perltrap.1

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\&
\&    package main;
\&    print "\e$_legacy is ",$_legacy,"\en";
\&
\&    # perl4 prints: $_legacy is 1
\&    # perl5 prints: $_legacy is
.Ve
.IP "\(bu" 4
Double-colon valid package separator in variable name
.Sp
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.
.Sp
.Vb 3
\&    $a=1;$b=2;$c=3;$var=4;
\&    print "$a::$b::$c ";
\&    print "$var::abc::xyz\en";
\&
\&    # perl4 prints: 1::2::3 4::abc::xyz
\&    # perl5 prints: 3
.Ve
.Sp
Given that \f(CW\*(C`::\*(C'\fR 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)
.Sp
.Vb 2
\&    $x = 10;
\&    print "x=${\*(Aqx}\en";
\&
\&    # perl4 prints: x=10
\&    # perl5 prints: Can\*(Aqt find string terminator "\*(Aq" anywhere before EOF
.Ve
.Sp
You can avoid this problem, and remain compatible with perl4, if you
always explicitly include the package name:
.Sp
.Vb 2
\&    $x = 10;
\&    print "x=${main\*(Aqx}\en";
.Ve
.Sp
Also see precedence traps, for parsing \f(CW$:\fR.
.IP "\(bu" 4
2nd and 3rd args to \f(CW\*(C`splice()\*(C'\fR are now in scalar context
.Sp
The second and third arguments of \f(CW\*(C`splice()\*(C'\fR are now evaluated in scalar
context (as the Camel says) rather than list context.
.Sp
.Vb 5
\&    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(\*(Aq \*(Aq,@a2),"\en";
\&
\&    # perl4 prints: a b
\&    # perl5 prints: c d e
.Ve
.IP "\(bu" 4
Can't do \f(CW\*(C`goto\*(C'\fR into a block that is optimized away
.Sp
You can't do a \f(CW\*(C`goto\*(C'\fR into a block that is optimized away.  Darn.
.Sp
.Vb 1
\&    goto marker1;
\&
\&    for(1){
\&    marker1:
\&        print "Here I is!\en";
\&    }
\&
\&    # perl4 prints: Here I is!
\&    # perl5 errors: Can\*(Aqt "goto" into the middle of a foreach loop
.Ve
.IP "\(bu" 4
Can't use whitespace as variable name or quote delimiter
.Sp
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.
.Sp
.Vb 3
\&    $a = ("foo bar");
\&    $b = q baz;
\&    print "a is $a, b is $b\en";
\&
\&    # perl4 prints: a is foo bar, b is baz
\&    # perl5 errors: Bareword found where operator expected
.Ve
.IP "\(bu" 4
\&\f(CW\*(C`while/if BLOCK BLOCK\*(C'\fR gone
.Sp
The archaic while/if \s-1BLOCK\s0 \s-1BLOCK\s0 syntax is no longer supported.
.Sp
.Vb 6
\&    if { 1 } {
\&        print "True!";
\&    }
\&    else {
\&        print "False!";
\&    }
\&
\&    # perl4 prints: True!
\&    # perl5 errors: syntax error at test.pl line 1, near "if {"
.Ve
.IP "\(bu" 4
\&\f(CW\*(C`**\*(C'\fR binds tighter than unary minus
.Sp
The \f(CW\*(C`**\*(C'\fR operator now binds more tightly than unary minus.
It was documented to work this way before, but didn't.
.Sp
.Vb 1
\&    print \-4**2,"\en";
\&
\&    # perl4 prints: 16
\&    # perl5 prints: \-16
.Ve
.IP "\(bu" 4
\&\f(CW\*(C`foreach\*(C'\fR changed when iterating over a list
.Sp
The meaning of \f(CW\*(C`foreach{}\*(C'\fR 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.
.Sp
.Vb 5
\&    @list = (\*(Aqab\*(Aq,\*(Aqabc\*(Aq,\*(Aqbcd\*(Aq,\*(Aqdef\*(Aq);
\&    foreach $var (grep(/ab/,@list)){
\&        $var = 1;
\&    }
\&    print (join(\*(Aq:\*(Aq,@list));
\&
\&    # perl4 prints: ab:abc:bcd:def
\&    # perl5 prints: 1:1:bcd:def
.Ve
.Sp
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
.Sp
.Vb 1
\&    foreach $var (grep(/ab/,@list)){
.Ve
.Sp
to
.Sp
.Vb 1
\&    foreach $var (@tmp = grep(/ab/,@list)){
.Ve
.Sp
Otherwise changing \f(CW$var\fR will clobber the values of \f(CW@list\fR.  (This most often
happens when you use \f(CW$_\fR for the loop variable, and call subroutines in
the loop that don't properly localize \f(CW$_\fR.)
.IP "\(bu" 4
\&\f(CW\*(C`split\*(C'\fR with no args behavior changed
.Sp
\&\f(CW\*(C`split\*(C'\fR with no arguments now behaves like \f(CW\*(C`split \*(Aq \*(Aq\*(C'\fR (which doesn't
return an initial null field if \f(CW$_\fR starts with whitespace), it used to
behave like \f(CW\*(C`split /\es+/\*(C'\fR (which does).
.Sp
.Vb 2
\&    $_ = \*(Aq hi mom\*(Aq;
\&    print join(\*(Aq:\*(Aq, split);
\&
\&    # perl4 prints: :hi:mom
\&    # perl5 prints: hi:mom
.Ve
.IP "\(bu" 4
\&\fB\-e\fR behavior fixed
.Sp
Perl 4 would ignore any text which was attached to an \fB\-e\fR switch,
always taking the code snippet from the following arg.  Additionally, it
would silently accept an \fB\-e\fR switch without a following arg.  Both of
these behaviors have been fixed.
.Sp
.Vb 1
\&    perl \-e\*(Aqprint "attached to \-e"\*(Aq \*(Aqprint "separate arg"\*(Aq
\&
\&    # perl4 prints: separate arg
\&    # perl5 prints: attached to \-e
\&
\&    perl \-e
\&
\&    # perl4 prints:
\&    # perl5 dies: No code specified for \-e.
.Ve
.IP "\(bu" 4
\&\f(CW\*(C`push\*(C'\fR returns number of elements in resulting list
.Sp
In Perl 4 the return value of \f(CW\*(C`push\*(C'\fR was undocumented, but it was
actually the last value being pushed onto the target list.  In Perl 5
the return value of \f(CW\*(C`push\*(C'\fR is documented, but has changed, it is the
number of elements in the resulting list.
.Sp
.Vb 2
\&    @x = (\*(Aqexisting\*(Aq);
\&    print push(@x, \*(Aqfirst new\*(Aq, \*(Aqsecond new\*(Aq);
\&
\&    # perl4 prints: second new
\&    # perl5 prints: 3
.Ve
.IP "\(bu" 4
Some error messages differ
.Sp
Some error messages will be different.
.IP "\(bu" 4
\&\f(CW\*(C`split()\*(C'\fR honors subroutine args
.Sp
In Perl 4, if in list context the delimiters to the first argument of
\&\f(CW\*(C`split()\*(C'\fR were \f(CW\*(C`??\*(C'\fR, the result would be placed in \f(CW@_\fR as well as
being returned.   Perl 5 has more respect for your subroutine arguments.
.IP "\(bu" 4
Bugs removed
.Sp
Some bugs may have been inadvertently removed.  :\-)
.Sh "Parsing Traps"
.IX Subsection "Parsing Traps"
Perl4\-to\-Perl5 traps from having to do with parsing.
.IP "\(bu" 4
Space between . and = triggers syntax error
.Sp
Note the space between . and =
.Sp
.Vb 2
\&    $string . = "more string";
\&    print $string;
\&
\&    # perl4 prints: more string
\&    # perl5 prints: syntax error at \- line 1, near ". ="
.Ve
.IP "\(bu" 4
Better parsing in perl 5
.Sp
Better parsing in perl 5
.Sp
.Vb 3
\&    sub foo {}
\&    &foo
\&    print("hello, world\en");
\&
\&    # perl4 prints: hello, world
\&    # perl5 prints: syntax error
.Ve
.IP "\(bu" 4
Function parsing
.Sp
\&\*(L"if it looks like a function, it is a function\*(R" rule.
.Sp
.Vb 2
\&  print
\&    ($foo == 1) ? "is one\en" : "is zero\en";
\&
\&    # perl4 prints: is zero
\&    # perl5 warns: "Useless use of a constant in void context" if using \-w
.Ve
.IP "\(bu" 4
String interpolation of \f(CW$#array\fR differs
.Sp
String interpolation of the \f(CW$#array\fR construct differs when braces
are to used around the name.
.Sp
.Vb 2
\&    @a = (1..3);
\&    print "${#a}";
\&
\&    # perl4 prints: 2
\&    # perl5 fails with syntax error
\&
\&    @ = (1..3);
\&    print "$#{a}";
\&
\&    # perl4 prints: {a}
\&    # perl5 prints: 2
.Ve
.IP "\(bu" 4
Perl guesses on \f(CW\*(C`map\*(C'\fR, \f(CW\*(C`grep\*(C'\fR followed by \f(CW\*(C`{\*(C'\fR if it starts \s-1BLOCK\s0 or hash ref
.Sp
When perl sees \f(CW\*(C`map {\*(C'\fR (or \f(CW\*(C`grep {\*(C'\fR), it has to guess whether the \f(CW\*(C`{\*(C'\fR
starts a \s-1BLOCK\s0 or a hash reference. If it guesses wrong, it will report
a syntax error near the \f(CW\*(C`}\*(C'\fR and the missing (or unexpected) comma.
.Sp
Use unary \f(CW\*(C`+\*(C'\fR before \f(CW\*(C`{\*(C'\fR on a hash reference, and unary \f(CW\*(C`+\*(C'\fR applied
to the first thing in a \s-1BLOCK\s0 (after \f(CW\*(C`{\*(C'\fR), for perl to guess right all
the time. (See \*(L"map\*(R" in perlfunc.)
.Sh "Numerical Traps"
.IX Subsection "Numerical Traps"
Perl4\-to\-Perl5 traps having to do with numerical operators,
operands, or output from same.
.IP "\(bu" 5
Formatted output and significant digits
.Sp
Formatted output and significant digits.  In general, Perl 5
tries to be more precise.  For example, on a Solaris Sparc:
.Sp
.Vb 2
\&    print 7.373504 \- 0, "\en";
\&    printf "%20.18f\en", 7.373504 \- 0;
\&
\&    # Perl4 prints:
\&    7.3750399999999996141
\&    7.375039999999999614
\&
\&    # Perl5 prints:
\&    7.373504
\&    7.375039999999999614
.Ve
.Sp
Notice how the first result looks better in Perl 5.
.Sp
Your results may vary, since your floating point formatting routines
and even floating point format may be slightly different.
.IP "\(bu" 5
Auto-increment operator over signed int limit deleted
.Sp
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:
.Sp
.Vb 1
\&   use Math::BigInt;
.Ve
.IP "\(bu" 5
Assignment of return values from numeric equality tests doesn't work
.Sp
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
.Sp
.Vb 2
\&    $p = ($test == 1);
\&    print $p,"\en";
\&
\&    # perl4 prints: 0
\&    # perl5 prints:
.Ve
.Sp
Also see \*(L"//, etc.\*(R"\*(L" in \*(R"General Regular Expression Traps using s
for another example of this new feature...
.IP "\(bu" 5
Bitwise string ops
.Sp
When bitwise operators which can operate upon either numbers or
strings (\f(CW\*(C`& | ^ ~\*(C'\fR) are given only strings as arguments, perl4 would
treat the operands as bitstrings so long as the program contained a call
to the \f(CW\*(C`vec()\*(C'\fR function. perl5 treats the string operands as bitstrings.
(See \*(L"Bitwise String Operators\*(R" in perlop for more details.)
.Sp
.Vb 6
\&    $fred = "10";
\&    $barney = "12";
\&    $betty = $fred & $barney;
\&    print "$betty\en";
\&    # Uncomment the next line to change perl4\*(Aqs behavior
\&    # ($dummy) = vec("dummy", 0, 0);
\&
\&    # Perl4 prints:
\&    8
\&
\&    # Perl5 prints:
\&    10
\&
\&    # If vec() is used anywhere in the program, both print:
\&    10
.Ve
.Sh "General data type traps"
.IX Subsection "General data type traps"
Perl4\-to\-Perl5 traps involving most data-types, and their usage
within certain expressions and/or context.
.IP "\(bu" 5
Negative array subscripts now count from the end of array
.Sp
Negative array subscripts now count from the end of the array.
.Sp
.Vb 2
\&    @a = (1, 2, 3, 4, 5);
\&    print "The third element of the array is $a[3] also expressed as $a[\-2] \en";
\&
\&    # 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
.Ve
.IP "\(bu" 5
Setting \f(CW$#array\fR lower now discards array elements
.Sp
Setting \f(CW$#array\fR lower now discards array elements, and makes them
impossible to recover.
.Sp
.Vb 6
\&    @a = (a,b,c,d,e);
\&    print "Before: ",join(\*(Aq\*(Aq,@a);
\&    $#a =1;
\&    print ", After: ",join(\*(Aq\*(Aq,@a);
\&    $#a =3;
\&    print ", Recovered: ",join(\*(Aq\*(Aq,@a),"\en";
\&
\&    # perl4 prints: Before: abcde, After: ab, Recovered: abcd
\&    # perl5 prints: Before: abcde, After: ab, Recovered: ab
.Ve
.IP "\(bu" 5
Hashes get defined before use
.Sp
Hashes get defined before use
.Sp
.Vb 4
\&    local($s,@a,%h);
\&    die "scalar \e$s defined" if defined($s);
\&    die "array \e@a defined" if defined(@a);
\&    die "hash \e%h defined" if defined(%h);
\&
\&    # perl4 prints:
\&    # perl5 dies: hash %h defined
.Ve
.Sp
Perl will now generate a warning when it sees defined(@a) and
defined(%h).
.IP "\(bu" 5
Glob assignment from localized variable to variable
.Sp
glob assignment from variable to variable will fail if the assigned
variable is localized subsequent to the assignment
.Sp