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PERLTRAP(1)	       Perl Programmers Reference Guide		   PERLTRAP(1)

NAME
       perltrap - Perl traps for the unwary

DESCRIPTION
       The biggest trap of all is forgetting to "use warnings" or use the -w
       switch; see perllexwarn and perlrun. The second biggest trap is not
       making your entire program runnable under "use strict".	The third
       biggest trap is not reading the list of changes in this version of
       Perl; see perldelta.

   Awk Traps
       Accustomed awk users should take special note of the following:

       ·   A Perl program executes only once, not once for each input line.
	   You can do an implicit loop with "-n" or "-p".

       ·   The English module, loaded via

	       use English;

	   allows you to refer to special variables (like $/) with names (like
	   $RS), as though they were in awk; see perlvar for details.

       ·   Semicolons are required after all simple statements in Perl (except
	   at the end of a block).  Newline is not a statement delimiter.

       ·   Curly brackets are required on "if"s and "while"s.

       ·   Variables begin with "$", "@" or "%" in Perl.

       ·   Arrays index from 0.	 Likewise string positions in substr() and
	   index().

       ·   You have to decide whether your array has numeric or string
	   indices.

       ·   Hash values do not spring into existence upon mere reference.

       ·   You have to decide whether you want to use string or numeric
	   comparisons.

       ·   Reading an input line does not split it for you.  You get to split
	   it to an array yourself.  And the split() operator has different
	   arguments than awk's.

       ·   The current input line is normally in $_, not $0.  It generally
	   does not have the newline stripped.	($0 is the name of the program
	   executed.)  See perlvar.

       ·   $<digit> does not refer to fields--it refers to substrings matched
	   by the last match pattern.

       ·   The print() statement does not add field and record separators
	   unless you set $, and "$\".	You can set $OFS and $ORS if you're
	   using the English module.

       ·   You must open your files before you print to them.

       ·   The range operator is "..", not comma.  The comma operator works as
	   in C.

       ·   The match operator is "=~", not "~".	 ("~" is the one's complement
	   operator, as in C.)

       ·   The exponentiation operator is "**", not "^".  "^" is the XOR
	   operator, as in C.  (You know, one could get the feeling that awk
	   is basically incompatible with C.)

       ·   The concatenation operator is ".", not the null string.  (Using the
	   null string would render "/pat/ /pat/" unparsable, because the
	   third slash would be interpreted as a division operator--the
	   tokenizer is in fact slightly context sensitive for operators like
	   "/", "?", and ">".  And in fact, "." itself can be the beginning of
	   a number.)

       ·   The "next", "exit", and "continue" keywords work differently.

       ·   The following variables work differently:

		 Awk	   Perl
		 ARGC	   scalar @ARGV (compare with $#ARGV)
		 ARGV[0]   $0
		 FILENAME  $ARGV
		 FNR	   $. - something
		 FS	   (whatever you like)
		 NF	   $#Fld, or some such
		 NR	   $.
		 OFMT	   $#
		 OFS	   $,
		 ORS	   $\
		 RLENGTH   length($&)
		 RS	   $/
		 RSTART	   length($`)
		 SUBSEP	   $;

       ·   You cannot set $RS to a pattern, only a string.

       ·   When in doubt, run the awk construct through a2p and see what it
	   gives you.

   C/C++ Traps
       Cerebral C and C++ programmers should take note of the following:

       ·   Curly brackets are required on "if"'s and "while"'s.

       ·   You must use "elsif" rather than "else if".

       ·   The "break" and "continue" keywords from C become in Perl "last"
	   and "next", respectively.  Unlike in C, these do not work within a
	   "do { } while" construct.  See "Loop Control" in perlsyn.

       ·   The switch statement is called "given/when" and only available in
	   perl 5.10 or newer. See "Switch statements" in perlsyn.

       ·   Variables begin with "$", "@" or "%" in Perl.

       ·   Comments begin with "#", not "/*" or "//".  Perl may interpret
	   C/C++ comments as division operators, unterminated regular
	   expressions or the defined-or operator.

       ·   You can't take the address of anything, although a similar operator
	   in Perl is the backslash, which creates a reference.

       ·   "ARGV" must be capitalized.	$ARGV[0] is C's "argv[1]", and
	   "argv[0]" ends up in $0.

       ·   System calls such as link(), unlink(), rename(), etc. return
	   nonzero for success, not 0. (system(), however, returns zero for
	   success.)

       ·   Signal handlers deal with signal names, not numbers.	 Use "kill -l"
	   to find their names on your system.

   Sed Traps
       Seasoned sed programmers should take note of the following:

       ·   A Perl program executes only once, not once for each input line.
	   You can do an implicit loop with "-n" or "-p".

       ·   Backreferences in substitutions use "$" rather than "\".

       ·   The pattern matching metacharacters "(", ")", and "|" do not have
	   backslashes in front.

       ·   The range operator is "...", rather than comma.

   Shell Traps
       Sharp shell programmers should take note of the following:

       ·   The backtick operator does variable interpolation without regard to
	   the presence of single quotes in the command.

       ·   The backtick operator does no translation of the return value,
	   unlike csh.

       ·   Shells (especially csh) do several levels of substitution on each
	   command line.  Perl does substitution in only certain constructs
	   such as double quotes, backticks, angle brackets, and search
	   patterns.

       ·   Shells interpret scripts a little bit at a time.  Perl compiles the
	   entire program before executing it (except for "BEGIN" blocks,
	   which execute at compile time).

       ·   The arguments are available via @ARGV, not $1, $2, etc.

       ·   The environment is not automatically made available as separate
	   scalar variables.

       ·   The shell's "test" uses "=", "!=", "<" etc for string comparisons
	   and "-eq", "-ne", "-lt" etc for numeric comparisons. This is the
	   reverse of Perl, which uses "eq", "ne", "lt" for string
	   comparisons, and "==", "!=" "<" etc for numeric comparisons.

   Perl Traps
       Practicing Perl Programmers should take note of the following:

       ·   Remember that many operations behave differently in a list context
	   than they do in a scalar one.  See perldata for details.

       ·   Avoid barewords if you can, especially all lowercase ones.  You
	   can't tell by just looking at it whether a bareword is a function
	   or a string.	 By using quotes on strings and parentheses on
	   function calls, you won't ever get them confused.

       ·   You cannot discern from mere inspection which builtins are unary
	   operators (like chop() and chdir()) and which are list operators
	   (like print() and unlink()).	 (Unless prototyped, user-defined
	   subroutines can only be list operators, never unary ones.)  See
	   perlop and perlsub.

       ·   People have a hard time remembering that some functions default to
	   $_, or @ARGV, or whatever, but that others which you might expect
	   to do not.

       ·   The <FH> construct is not the name of the filehandle, it is a
	   readline operation on that handle.  The data read is assigned to $_
	   only if the file read is the sole condition in a while loop:

	       while (<FH>)	 { }
	       while (defined($_ = <FH>)) { }..
	       <FH>;  # data discarded!

       ·   Remember not to use "=" when you need "=~"; these two constructs
	   are quite different:

	       $x =  /foo/;
	       $x =~ /foo/;

       ·   The "do {}" construct isn't a real loop that you can use loop
	   control on.

       ·   Use "my()" for local variables whenever you can get away with it
	   (but see perlform for where you can't).  Using "local()" actually
	   gives a local value to a global variable, which leaves you open to
	   unforeseen side-effects of dynamic scoping.

       ·   If you localize an exported variable in a module, its exported
	   value will not change.  The local name becomes an alias to a new
	   value but the external name is still an alias for the original.

   Perl4 to Perl5 Traps
       Practicing Perl4 Programmers should take note of the following
       Perl4-to-Perl5 specific traps.

       They're crudely ordered according to the following list:

       Discontinuance, Deprecation, and BugFix traps
	   Anything that's been fixed as a perl4 bug, removed as a perl4
	   feature or deprecated as a perl4 feature with the intent to
	   encourage usage of some other perl5 feature.

       Parsing Traps
	   Traps that appear to stem from the new parser.

       Numerical Traps
	   Traps having to do with numerical or mathematical operators.

       General data type traps
	   Traps involving perl standard data types.

       Context Traps - scalar, list contexts
	   Traps related to context within lists, scalar
	   statements/declarations.

       Precedence Traps
	   Traps related to the precedence of parsing, evaluation, and
	   execution of code.

       General Regular Expression Traps using s///, etc.
	   Traps related to the use of pattern matching.

       Subroutine, Signal, Sorting Traps
	   Traps related to the use of signals and signal handlers, general
	   subroutines, and sorting, along with sorting subroutines.

       OS Traps
	   OS-specific traps.

       DBM Traps
	   Traps specific to the use of "dbmopen()", and specific dbm
	   implementations.

       Unclassified Traps
	   Everything else.

       If you find an example of a conversion trap that is not listed here,
       please submit it to <perlbug@perl.org> for inclusion.  Also note that
       at least some of these can be caught with the "use warnings" pragma or
       the -w switch.

   Discontinuance, Deprecation, and BugFix traps
       Anything that has been discontinued, deprecated, or fixed as a bug from
       perl4.

       ·   Symbols starting with "_" no longer forced into main

	   Symbols starting with "_" are no longer forced into package main,
	   except for $_ itself (and @_, etc.).

	       package test;
	       $_legacy = 1;

	       package main;
	       print "\$_legacy is ",$_legacy,"\n";

	       # perl4 prints: $_legacy is 1
	       # perl5 prints: $_legacy is

       ·   Double-colon valid package separator in variable name

	   Double-colon is now a valid package separator in a variable name.
	   Thus these behave differently in perl4 vs. perl5, because the
	   packages don't exist.

	       $a=1;$b=2;$c=3;$var=4;
	       print "$a::$b::$c ";
	       print "$var::abc::xyz\n";

	       # perl4 prints: 1::2::3 4::abc::xyz
	       # perl5 prints: 3

	   Given that "::" is now the preferred package delimiter, it is
	   debatable whether this should be classed as a bug or not.  (The
	   older package delimiter, ' ,is used here)

	       $x = 10;
	       print "x=${'x}\n";

	       # perl4 prints: x=10
	       # perl5 prints: Can't find string terminator "'" anywhere before EOF

	   You can avoid this problem, and remain compatible with perl4, if
	   you always explicitly include the package name:

	       $x = 10;
	       print "x=${main'x}\n";

	   Also see precedence traps, for parsing $:.

       ·   2nd and 3rd args to "splice()" are now in scalar context

	   The second and third arguments of "splice()" are now evaluated in
	   scalar context (as the Camel says) rather than list context.

	       sub sub1{return(0,2) }	       # return a 2-element list
	       sub sub2{ return(1,2,3)}	       # return a 3-element list
	       @a1 = ("a","b","c","d","e");
	       @a2 = splice(@a1,&sub1,&sub2);
	       print join(' ',@a2),"\n";

	       # perl4 prints: a b
	       # perl5 prints: c d e

       ·   Can't do "goto" into a block that is optimized away

	   You can't do a "goto" into a block that is optimized away.  Darn.

	       goto marker1;

	       for(1){
	       marker1:
		   print "Here I is!\n";
	       }

	       # perl4 prints: Here I is!
	       # perl5 errors: Can't "goto" into the middle of a foreach loop

       ·   Can't use whitespace as variable name or quote delimiter

	   It is no longer syntactically legal to use whitespace as the name
	   of a variable, or as a delimiter for any kind of quote construct.
	   Double darn.

	       $a = ("foo bar");
	       $b = q baz;
	       print "a is $a, b is $b\n";

	       # perl4 prints: a is foo bar, b is baz
	       # perl5 errors: Bareword found where operator expected

       ·   "while/if BLOCK BLOCK" gone

	   The archaic while/if BLOCK BLOCK syntax is no longer supported.

	       if { 1 } {
		   print "True!";
	       }
	       else {
		   print "False!";
	       }

	       # perl4 prints: True!
	       # perl5 errors: syntax error at test.pl line 1, near "if {"

       ·   "**" binds tighter than unary minus

	   The "**" operator now binds more tightly than unary minus.  It was
	   documented to work this way before, but didn't.

	       print -4**2,"\n";

	       # perl4 prints: 16
	       # perl5 prints: -16

       ·   "foreach" changed when iterating over a list

	   The meaning of "foreach{}" has changed slightly when it is
	   iterating over a list which is not an array.	 This used to assign
	   the list to a temporary array, but no longer does so (for
	   efficiency).	 This means that you'll now be iterating over the
	   actual values, not over copies of the values.  Modifications to the
	   loop variable can change the original values.

	       @list = ('ab','abc','bcd','def');
	       foreach $var (grep(/ab/,@list)){
		   $var = 1;
	       }
	       print (join(':',@list));

	       # perl4 prints: ab:abc:bcd:def
	       # perl5 prints: 1:1:bcd:def

	   To retain Perl4 semantics you need to assign your list explicitly
	   to a temporary array and then iterate over that.  For example, you
	   might need to change

	       foreach $var (grep(/ab/,@list)){

	   to

	       foreach $var (@tmp = grep(/ab/,@list)){

	   Otherwise changing $var will clobber the values of @list.  (This
	   most often happens when you use $_ for the loop variable, and call
	   subroutines in the loop that don't properly localize $_.)

       ·   "split" with no args behavior changed

	   "split" with no arguments now behaves like "split ' '" (which
	   doesn't return an initial null field if $_ starts with whitespace),
	   it used to behave like "split /\s+/" (which does).

	       $_ = ' hi mom';
	       print join(':', split);

	       # perl4 prints: :hi:mom
	       # perl5 prints: hi:mom

       ·   -e behavior fixed

	   Perl 4 would ignore any text which was attached to an -e switch,
	   always taking the code snippet from the following arg.
	   Additionally, it would silently accept an -e switch without a
	   following arg.  Both of these behaviors have been fixed.

	       perl -e'print "attached to -e"' 'print "separate arg"'

	       # perl4 prints: separate arg
	       # perl5 prints: attached to -e

	       perl -e

	       # perl4 prints:
	       # perl5 dies: No code specified for -e.

       ·   "push" returns number of elements in resulting list

	   In Perl 4 the return value of "push" was undocumented, but it was
	   actually the last value being pushed onto the target list.  In Perl
	   5 the return value of "push" is documented, but has changed, it is
	   the number of elements in the resulting list.

	       @x = ('existing');
	       print push(@x, 'first new', 'second new');

	       # perl4 prints: second new
	       # perl5 prints: 3

       ·   Some error messages differ

	   Some error messages will be different.

       ·   "split()" honors subroutine args

	   In Perl 4, if in list context the delimiters to the first argument
	   of "split()" were "??", the result would be placed in @_ as well as
	   being returned.   Perl 5 has more respect for your subroutine
	   arguments.

       ·   Bugs removed

	   Some bugs may have been inadvertently removed.  :-)

   Parsing Traps
       Perl4-to-Perl5 traps from having to do with parsing.

       ·   Space between . and = triggers syntax error

	   Note the space between . and =

	       $string . = "more string";
	       print $string;

	       # perl4 prints: more string
	       # perl5 prints: syntax error at - line 1, near ". ="

       ·   Better parsing in perl 5

	   Better parsing in perl 5

	       sub foo {}
	       &foo
	       print("hello, world\n");

	       # perl4 prints: hello, world
	       # perl5 prints: syntax error

       ·   Function parsing

	   "if it looks like a function, it is a function" rule.

	     print
	       ($foo == 1) ? "is one\n" : "is zero\n";

	       # perl4 prints: is zero
	       # perl5 warns: "Useless use of a constant in void context" if using -w

       ·   String interpolation of $#array differs

	   String interpolation of the $#array construct differs when braces
	   are to used around the name.

	       @a = (1..3);
	       print "${#a}";

	       # perl4 prints: 2
	       # perl5 fails with syntax error

	       @ = (1..3);
	       print "$#{a}";

	       # perl4 prints: {a}
	       # perl5 prints: 2

       ·   Perl guesses on "map", "grep" followed by "{" if it starts BLOCK or
	   hash ref

	   When perl sees "map {" (or "grep {"), it has to guess whether the
	   "{" starts a BLOCK or a hash reference. If it guesses wrong, it
	   will report a syntax error near the "}" and the missing (or
	   unexpected) comma.

	   Use unary "+" before "{" on a hash reference, and unary "+" applied
	   to the first thing in a BLOCK (after "{"), for perl to guess right
	   all the time. (See "map" in perlfunc.)

   Numerical Traps
       Perl4-to-Perl5 traps having to do with numerical operators, operands,
       or output from same.

       ·    Formatted output and significant digits

	    Formatted output and significant digits.  In general, Perl 5 tries
	    to be more precise.	 For example, on a Solaris Sparc:

		print 7.373504 - 0, "\n";
		printf "%20.18f\n", 7.373504 - 0;

		# Perl4 prints:
		7.3750399999999996141
		7.375039999999999614

		# Perl5 prints:
		7.373504
		7.375039999999999614

	    Notice how the first result looks better in Perl 5.

	    Your results may vary, since your floating point formatting
	    routines and even floating point format may be slightly different.

       ·    Auto-increment operator over signed int limit deleted

	    This specific item has been deleted.  It demonstrated how the
	    auto-increment operator would not catch when a number went over
	    the signed int limit.  Fixed in version 5.003_04.  But always be
	    wary when using large integers.  If in doubt:

	       use Math::BigInt;

       ·    Assignment of return values from numeric equality tests doesn't
	    work

	    Assignment of return values from numeric equality tests does not
	    work in perl5 when the test evaluates to false (0).	 Logical tests
	    now return a null, instead of 0

		$p = ($test == 1);
		print $p,"\n";

		# perl4 prints: 0
		# perl5 prints:

	    Also see "//, etc."" in "General Regular Expression Traps using s
	    for another example of this new feature...

       ·    Bitwise string ops

	    When bitwise operators which can operate upon either numbers or
	    strings ("& | ^ ~") are given only strings as arguments, perl4
	    would treat the operands as bitstrings so long as the program
	    contained a call to the "vec()" function. perl5 treats the string
	    operands as bitstrings.  (See "Bitwise String Operators" in perlop
	    for more details.)

		$fred = "10";
		$barney = "12";
		$betty = $fred & $barney;
		print "$betty\n";
		# Uncomment the next line to change perl4's behavior
		# ($dummy) = vec("dummy", 0, 0);

		# Perl4 prints:
		8

		# Perl5 prints:
		10

		# If vec() is used anywhere in the program, both print:
		10

   General data type traps
       Perl4-to-Perl5 traps involving most data-types, and their usage within
       certain expressions and/or context.

       ·    Negative array subscripts now count from the end of array

	    Negative array subscripts now count from the end of the array.

		@a = (1, 2, 3, 4, 5);
		print "The third element of the array is $a[3] also expressed as $a[-2] \n";

		# perl4 prints: The third element of the array is 4 also expressed as
		# perl5 prints: The third element of the array is 4 also expressed as 4

       ·    Setting $#array lower now discards array elements

	    Setting $#array lower now discards array elements, and makes them
	    impossible to recover.

		@a = (a,b,c,d,e);
		print "Before: ",join('',@a);
		$#a =1;
		print ", After: ",join('',@a);
		$#a =3;
		print ", Recovered: ",join('',@a),"\n";

		# perl4 prints: Before: abcde, After: ab, Recovered: abcd
		# perl5 prints: Before: abcde, After: ab, Recovered: ab

       ·    Hashes get defined before use

	    Hashes get defined before use

		local($s,@a,%h);
		die "scalar \$s defined" if defined($s);
		die "array \@a defined" if defined(@a);
		die "hash \%h defined" if defined(%h);

		# perl4 prints:
		# perl5 dies: hash %h defined

	    Perl will now generate a warning when it sees defined(@a) and
	    defined(%h).

       ·    Glob assignment from localized variable to variable

	    glob assignment from variable to variable will fail if the
	    assigned variable is localized subsequent to the assignment

		@a = ("This is Perl 4");
		*b = *a;
		local(@a);
		print @b,"\n";

		# perl4 prints: This is Perl 4
		# perl5 prints:

       ·    Assigning "undef" to glob

	    Assigning "undef" to a glob has no effect in Perl 5.   In Perl 4
	    it undefines the associated scalar (but may have other side
	    effects including SEGVs). Perl 5 will also warn if "undef" is
	    assigned to a typeglob. (Note that assigning "undef" to a typeglob
	    is different than calling the "undef" function on a typeglob
	    ("undef *foo"), which has quite a few effects.

		$foo = "bar";
		*foo = undef;
		print $foo;

		# perl4 prints:
		# perl4 warns: "Use of uninitialized variable" if using -w
		# perl5 prints: bar
		# perl5 warns: "Undefined value assigned to typeglob" if using -w

       ·    Changes in unary negation (of strings)

	    Changes in unary negation (of strings) This change effects both
	    the return value and what it does to auto(magic)increment.

		$x = "aaa";
		print ++$x," : ";
		print -$x," : ";
		print ++$x,"\n";

		# perl4 prints: aab : -0 : 1
		# perl5 prints: aab : -aab : aac

       ·    Modifying of constants prohibited

	    perl 4 lets you modify constants:

		$foo = "x";
		&mod($foo);
		for ($x = 0; $x < 3; $x++) {
		    &mod("a");
		}
		sub mod {
		    print "before: $_[0]";
		    $_[0] = "m";
		    print "  after: $_[0]\n";
		}

		# perl4:
		# before: x  after: m
		# before: a  after: m
		# before: m  after: m
		# before: m  after: m

		# Perl5:
		# before: x  after: m
		# Modification of a read-only value attempted at foo.pl line 12.
		# before: a

       ·    "defined $var" behavior changed

	    The behavior is slightly different for:

		print "$x", defined $x

		# perl 4: 1
		# perl 5: <no output, $x is not called into existence>

       ·    Variable Suicide

	    Variable suicide behavior is more consistent under Perl 5.	Perl5
	    exhibits the same behavior for hashes and scalars, that perl4
	    exhibits for only scalars.

		$aGlobal{ "aKey" } = "global value";
		print "MAIN:", $aGlobal{"aKey"}, "\n";
		$GlobalLevel = 0;
		&test( *aGlobal );

		sub test {
		    local( *theArgument ) = @_;
		    local( %aNewLocal ); # perl 4 != 5.001l,m
		    $aNewLocal{"aKey"} = "this should never appear";
		    print "SUB: ", $theArgument{"aKey"}, "\n";
		    $aNewLocal{"aKey"} = "level $GlobalLevel";	 # what should print
		    $GlobalLevel++;
		    if( $GlobalLevel<4 ) {
			&test( *aNewLocal );
		    }
		}

		# Perl4:
		# MAIN:global value
		# SUB: global value
		# SUB: level 0
		# SUB: level 1
		# SUB: level 2

		# Perl5:
		# MAIN:global value
		# SUB: global value
		# SUB: this should never appear
		# SUB: this should never appear
		# SUB: this should never appear

   Context Traps - scalar, list contexts
       ·    Elements of argument lists for formats evaluated in list context

	    The elements of argument lists for formats are now evaluated in
	    list context.  This means you can interpolate list values now.

		@fmt = ("foo","bar","baz");
		format STDOUT=
		@<<<<< @||||| @>>>>>
		@fmt;
		.
		write;

		# perl4 errors:	 Please use commas to separate fields in file
		# perl5 prints: foo	bar	 baz

       ·    "caller()" returns false value in scalar context if no caller
	    present

	    The "caller()" function now returns a false value in a scalar
	    context if there is no caller.  This lets library files determine
	    if they're being required.

		caller() ? (print "You rang?\n") : (print "Got a 0\n");

		# perl4 errors: There is no caller
		# perl5 prints: Got a 0

       ·    Comma operator in scalar context gives scalar context to args

	    The comma operator in a scalar context is now guaranteed to give a
	    scalar context to its arguments.

		@y= ('a','b','c');
		$x = (1, 2, @y);
		print "x = $x\n";

		# Perl4 prints:	 x = c	 # Thinks list context interpolates list
		# Perl5 prints:	 x = 3	 # Knows scalar uses length of list

       ·    "sprintf()" prototyped as "($;@)"

	    "sprintf()" is prototyped as ($;@), so its first argument is given
	    scalar context. Thus, if passed an array, it will probably not do
	    what you want, unlike Perl 4:

		@z = ('%s%s', 'foo', 'bar');
		$x = sprintf(@z);
		print $x;

		# perl4 prints: foobar
		# perl5 prints: 3

	    "printf()" works the same as it did in Perl 4, though:

		@z = ('%s%s', 'foo', 'bar');
		printf STDOUT (@z);

		# perl4 prints: foobar
		# perl5 prints: foobar

   Precedence Traps
       Perl4-to-Perl5 traps involving precedence order.

       Perl 4 has almost the same precedence rules as Perl 5 for the operators
       that they both have.  Perl 4 however, seems to have had some
       inconsistencies that made the behavior differ from what was documented.

       ·    LHS vs. RHS of any assignment operator

	    LHS vs. RHS of any assignment operator.  LHS is evaluated first in
	    perl4, second in perl5; this can affect the relationship between
	    side-effects in sub-expressions.

		@arr = ( 'left', 'right' );
		$a{shift @arr} = shift @arr;
		print join( ' ', keys %a );

		# perl4 prints: left
		# perl5 prints: right

       ·    Semantic errors introduced due to precedence

	    These are now semantic errors because of precedence:

		@list = (1,2,3,4,5);
		%map = ("a",1,"b",2,"c",3,"d",4);
		$n = shift @list + 2;	# first item in list plus 2
		print "n is $n, ";
		$m = keys %map + 2;	# number of items in hash plus 2
		print "m is $m\n";

		# perl4 prints: n is 3, m is 6
		# perl5 errors and fails to compile

       ·    Precedence of assignment operators same as the precedence of
	    assignment

	    The precedence of assignment operators is now the same as the
	    precedence of assignment.  Perl 4 mistakenly gave them the
	    precedence of the associated operator.  So you now must
	    parenthesize them in expressions like

		/foo/ ? ($a += 2) : ($a -= 2);

	    Otherwise

		/foo/ ? $a += 2 : $a -= 2

	    would be erroneously parsed as

		(/foo/ ? $a += 2 : $a) -= 2;

	    On the other hand,

		$a += /foo/ ? 1 : 2;

	    now works as a C programmer would expect.

       ·    "open" requires parentheses around filehandle

		open FOO || die;

	    is now incorrect.  You need parentheses around the filehandle.
	    Otherwise, perl5 leaves the statement as its default precedence:

		open(FOO || die);

		# perl4 opens or dies
		# perl5 opens FOO, dying only if 'FOO' is false, i.e. never

       ·    $: precedence over $:: gone

	    perl4 gives the special variable, $: precedence, where perl5
	    treats $:: as main "package"

		$a = "x"; print "$::a";

		# perl 4 prints: -:a
		# perl 5 prints: x

       ·    Precedence of file test operators documented

	    perl4 had buggy precedence for the file test operators vis-a-vis
	    the assignment operators.  Thus, although the precedence table for
	    perl4 leads one to believe "-e $foo .= "q"" should parse as "((-e
	    $foo) .= "q")", it actually parses as "(-e ($foo .= "q"))".	 In
	    perl5, the precedence is as documented.

		-e $foo .= "q"

		# perl4 prints: no output
		# perl5 prints: Can't modify -e in concatenation

       ·    "keys", "each", "values" are regular named unary operators

	    In perl4, keys(), each() and values() were special high-precedence
	    operators that operated on a single hash, but in perl5, they are
	    regular named unary operators.  As documented, named unary
	    operators have lower precedence than the arithmetic and
	    concatenation operators "+ - .", but the perl4 variants of these
	    operators actually bind tighter than "+ - .".  Thus, for:

		%foo = 1..10;
		print keys %foo - 1

		# perl4 prints: 4
		# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)

	    The perl4 behavior was probably more useful, if less consistent.

   General Regular Expression Traps using s///, etc.
       All types of RE traps.

       ·    "s'$lhs'$rhs'" interpolates on either side

	    "s'$lhs'$rhs'" now does no interpolation on either side.  It used
	    to interpolate $lhs but not $rhs.  (And still does not match a
	    literal '$' in string)

		$a=1;$b=2;
		$string = '1 2 $a $b';
		$string =~ s'$a'$b';
		print $string,"\n";

		# perl4 prints: $b 2 $a $b
		# perl5 prints: 1 2 $a $b

       ·    "m//g" attaches its state to the searched string

	    "m//g" now attaches its state to the searched string rather than
	    the regular expression.  (Once the scope of a block is left for
	    the sub, the state of the searched string is lost)

		$_ = "ababab";
		while(m/ab/g){
		    &doit("blah");
		}
		sub doit{local($_) = shift; print "Got $_ "}

		# perl4 prints: Got blah Got blah Got blah Got blah
		# perl5 prints: infinite loop blah...

       ·    "m//o" used within an anonymous sub

	    Currently, if you use the "m//o" qualifier on a regular expression
	    within an anonymous sub, all closures generated from that
	    anonymous sub will use the regular expression as it was compiled
	    when it was used the very first time in any such closure.  For
	    instance, if you say

		sub build_match {
		    my($left,$right) = @_;
		    return sub { $_[0] =~ /$left stuff $right/o; };
		}
		$good = build_match('foo','bar');
		$bad = build_match('baz','blarch');
		print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
		print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
		print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";

	    For most builds of Perl5, this will print: ok not ok not ok

	    build_match() will always return a sub which matches the contents
	    of $left and $right as they were the first time that build_match()
	    was called, not as they are in the current call.

       ·    $+ isn't set to whole match

	    If no parentheses are used in a match, Perl4 sets $+ to the whole
	    match, just like $&. Perl5 does not.

		"abcdef" =~ /b.*e/;
		print "\$+ = $+\n";

		# perl4 prints: bcde
		# perl5 prints:

       ·    Substitution now returns null string if it fails

	    substitution now returns the null string if it fails

		$string = "test";
		$value = ($string =~ s/foo//);
		print $value, "\n";

		# perl4 prints: 0
		# perl5 prints:

	    Also see "Numerical Traps" for another example of this new
	    feature.

       ·    "s`lhs`rhs`" is now a normal substitution

	    "s`lhs`rhs`" (using backticks) is now a normal substitution, with
	    no backtick expansion

		$string = "";
		$string =~ s`^`hostname`;
		print $string, "\n";

		# perl4 prints: <the local hostname>
		# perl5 prints: hostname

       ·    Stricter parsing of variables in regular expressions

	    Stricter parsing of variables used in regular expressions

		s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;

		# perl4: compiles w/o error
		# perl5: with Scalar found where operator expected ..., near "$opt$plus"

	    an added component of this example, apparently from the same
	    script, is the actual value of the s'd string after the
	    substitution.  "[$opt]" is a character class in perl4 and an array
	    subscript in perl5

		$grpc = 'a';
		$opt  = 'r';
		$_ = 'bar';
		s/^([^$grpc]*$grpc[$opt]?)/foo/;
		print;

		# perl4 prints: foo
		# perl5 prints: foobar

       ·    "m?x?" matches only once

	    Under perl5, "m?x?" matches only once, like "?x?". Under perl4, it
	    matched repeatedly, like "/x/" or "m!x!".

		$test = "once";
		sub match { $test =~ m?once?; }
		&match();
		if( &match() ) {
		    # m?x? matches more then once
		    print "perl4\n";
		} else {
		    # m?x? matches only once
		    print "perl5\n";
		}

		# perl4 prints: perl4
		# perl5 prints: perl5

       ·    Failed matches don't reset the match variables

	    Unlike in Ruby, failed matches in Perl do not reset the match
	    variables ($1, $2, ..., "$`", ...).

   Subroutine, Signal, Sorting Traps
       The general group of Perl4-to-Perl5 traps having to do with Signals,
       Sorting, and their related subroutines, as well as general subroutine
       traps.  Includes some OS-Specific traps.

       ·    Barewords that used to look like strings look like subroutine
	    calls

	    Barewords that used to look like strings to Perl will now look
	    like subroutine calls if a subroutine by that name is defined
	    before the compiler sees them.

		sub SeeYa { warn"Hasta la vista, baby!" }
		$SIG{'TERM'} = SeeYa;
		print "SIGTERM is now $SIG{'TERM'}\n";

		# perl4 prints: SIGTERM is now main'SeeYa
		# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")

	    Use -w to catch this one

       ·    Reverse is no longer allowed as the name of a sort subroutine

	    reverse is no longer allowed as the name of a sort subroutine.

		sub reverse{ print "yup "; $a <=> $b }
		print sort reverse (2,1,3);

		# perl4 prints: yup yup 123
		# perl5 prints: 123
		# perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()

       ·    "warn()" won't let you specify a filehandle.

	    Although it _always_ printed to STDERR, warn() would let you
	    specify a filehandle in perl4.  With perl5 it does not.

		warn STDERR "Foo!";

		# perl4 prints: Foo!
		# perl5 prints: String found where operator expected

   OS Traps
       ·    SysV resets signal handler correctly

	    Under HPUX, and some other SysV OSes, one had to reset any signal
	    handler, within  the signal handler function, each time a signal
	    was handled with perl4.  With perl5, the reset is now done
	    correctly.	Any code relying on the handler _not_ being reset will
	    have to be reworked.

	    Since version 5.002, Perl uses sigaction() under SysV.

		sub gotit {
		    print "Got @_... ";
		}
		$SIG{'INT'} = 'gotit';

		$| = 1;
		$pid = fork;
		if ($pid) {
		    kill('INT', $pid);
		    sleep(1);
		    kill('INT', $pid);
		} else {
		    while (1) {sleep(10);}
		}

		# perl4 (HPUX) prints: Got INT...
		# perl5 (HPUX) prints: Got INT... Got INT...

       ·    SysV "seek()" appends correctly

	    Under SysV OSes, "seek()" on a file opened to append ">>" now does
	    the right thing w.r.t. the fopen() manpage. e.g., - When a file is
	    opened for append,	it  is	impossible to overwrite information
	    already in the file.

		open(TEST,">>seek.test");
		$start = tell TEST;
		foreach(1 .. 9){
		    print TEST "$_ ";
		}
		$end = tell TEST;
		seek(TEST,$start,0);
		print TEST "18 characters here";

		# perl4 (solaris) seek.test has: 18 characters here
		# perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here

   Interpolation Traps
       Perl4-to-Perl5 traps having to do with how things get interpolated
       within certain expressions, statements, contexts, or whatever.

       ·    "@" always interpolates an array in double-quotish strings

	    @ now always interpolates an array in double-quotish strings.

		print "To: someone@somewhere.com\n";

		# perl4 prints: To:someone@somewhere.com
		# perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
		# perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string

       ·    Double-quoted strings may no longer end with an unescaped $

	    Double-quoted strings may no longer end with an unescaped $.

		$foo = "foo$";
		print "foo is $foo\n";

		# perl4 prints: foo is foo$
		# perl5 errors: Final $ should be \$ or $name

	    Note: perl5 DOES NOT error on the terminating @ in $bar

       ·    Arbitrary expressions are evaluated inside braces within double
	    quotes

	    Perl now sometimes evaluates arbitrary expressions inside braces
	    that occur within double quotes (usually when the opening brace is
	    preceded by "$" or "@").

		@www = "buz";
		$foo = "foo";
		$bar = "bar";
		sub foo { return "bar" };
		print "|@{w.w.w}|${main'foo}|";

		# perl4 prints: |@{w.w.w}|foo|
		# perl5 prints: |buz|bar|

	    Note that you can "use strict;" to ward off such trappiness under
	    perl5.

       ·    $$x now tries to dereference $x

	    The construct "this is $$x" used to interpolate the pid at that
	    point, but now tries to dereference $x.  $$ by itself still works
	    fine, however.

		$s = "a reference";
		$x = *s;
		print "this is $$x\n";

		# perl4 prints: this is XXXx   (XXX is the current pid)
		# perl5 prints: this is a reference

       ·    Creation of hashes on the fly with "eval "EXPR"" requires
	    protection

	    Creation of hashes on the fly with "eval "EXPR"" now requires
	    either both "$"'s to be protected in the specification of the hash
	    name, or both curlies to be protected.  If both curlies are
	    protected, the result will be compatible with perl4 and perl5.
	    This is a very common practice, and should be changed to use the
	    block form of "eval{}"  if possible.

		$hashname = "foobar";
		$key = "baz";
		$value = 1234;
		eval "\$$hashname{'$key'} = q|$value|";
		(defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");

		# perl4 prints: Yup
		# perl5 prints: Nope

	    Changing

		eval "\$$hashname{'$key'} = q|$value|";

	    to

		eval "\$\$hashname{'$key'} = q|$value|";

	    causes the following result:

		# perl4 prints: Nope
		# perl5 prints: Yup

	    or, changing to

		eval "\$$hashname\{'$key'\} = q|$value|";

	    causes the following result:

		# perl4 prints: Yup
		# perl5 prints: Yup
		# and is compatible for both versions

       ·    Bugs in earlier perl versions

	    perl4 programs which unconsciously rely on the bugs in earlier
	    perl versions.

		perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'

		# perl4 prints: This is not perl5
		# perl5 prints: This is perl5

       ·    Array and hash brackets during interpolation

	    You also have to be careful about array and hash brackets during
	    interpolation.

		print "$foo["

		perl 4 prints: [
		perl 5 prints: syntax error

		print "$foo{"

		perl 4 prints: {
		perl 5 prints: syntax error

	    Perl 5 is expecting to find an index or key name following the
	    respective brackets, as well as an ending bracket of the
	    appropriate type.  In order to mimic the behavior of Perl 4, you
	    must escape the bracket like so.

		print "$foo\[";
		print "$foo\{";

       ·    Interpolation of "\$$foo{bar}"

	    Similarly, watch out for: "\$$foo{bar}"

		$foo = "baz";
		print "\$$foo{bar}\n";

		# perl4 prints: $baz{bar}
		# perl5 prints: $

	    Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 is
	    happy just to expand $foo to "baz" by itself.  Watch out for this
	    especially in "eval"'s.

       ·    "qq()" string passed to "eval" will not find string terminator

	    "qq()" string passed to "eval"

		eval qq(
		    foreach \$y (keys %\$x\) {
			\$count++;
		    }
		);

		# perl4 runs this ok
		# perl5 prints: Can't find string terminator ")"

   DBM Traps
       General DBM traps.

       ·    Perl5 must have been linked with same dbm/ndbm as the default for
	    "dbmopen()"

	    Existing dbm databases created under perl4 (or any other dbm/ndbm
	    tool) may cause the same script, run under perl5, to fail.	The
	    build of perl5 must have been linked with the same dbm/ndbm as the
	    default for "dbmopen()" to function properly without "tie"'ing to
	    an extension dbm implementation.

		dbmopen (%dbm, "file", undef);
		print "ok\n";

		# perl4 prints: ok
		# perl5 prints: ok (IFF linked with -ldbm or -lndbm)

       ·    DBM exceeding limit on the key/value size will cause perl5 to exit
	    immediately

	    Existing dbm databases created under perl4 (or any other dbm/ndbm
	    tool) may cause the same script, run under perl5, to fail.	The
	    error generated when exceeding the limit on the key/value size
	    will cause perl5 to exit immediately.

		dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
		$DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
		print "YUP\n";

		# perl4 prints:
		dbm store returned -1, errno 28, key "trap" at - line 3.
		YUP

		# perl5 prints:
		dbm store returned -1, errno 28, key "trap" at - line 3.

   Unclassified Traps
       Everything else.

       ·    "require"/"do" trap using returned value

	    If the file doit.pl has:

		sub foo {
		    $rc = do "./do.pl";
		    return 8;
		}
		print &foo, "\n";

	    And the do.pl file has the following single line:

		return 3;

	    Running doit.pl gives the following:

		# perl 4 prints: 3 (aborts the subroutine early)
		# perl 5 prints: 8

	    Same behavior if you replace "do" with "require".

       ·    "split" on empty string with LIMIT specified

		$string = '';
		@list = split(/foo/, $string, 2)

	    Perl4 returns a one element list containing the empty string but
	    Perl5 returns an empty list.

       As always, if any of these are ever officially declared as bugs,
       they'll be fixed and removed.

perl v5.10.1			  2009-07-16			   PERLTRAP(1)
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