See perlsyn, specifically the section on Statement Modifiers:

NOTE: The behaviour of a my statement modified with a statement modifier conditional or loop construct (e.g. my $x if ...) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons.

Thanks! I couldn't find it before, but I see it now in perlsyn.

This is the ever-popular "static local variable" thing. The variable really is local - if you try to access @a outside fn you will get an error.

The explanation, as I understand it, is that the action of the my in creating the variable is a one-time (compile-time, in fact) action so you get the same variable every time. But the assignment part is run-time and is skipped because of the modifier. This doesn't quite make sense to me, though, when you think about recursion, closures, etc.

IMHO it would be nice to have an explicit feature for this, but it seems unlikely in Perl 5. Meanwhile, you can do the equivalent on your own:

{
   my @a;
   sub fn {
        # code that handles @a
   }
}
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5.10 throws a warning for that type of construct

No it doesn't. I seem to recall that blead did, at some point, but it appears to no longer do so.

It has changed the behaviour, now the assignment just becomes undefined, but the previous value is not recovered from the pad.

#! /usr/local/bin/perl5.9.5

use strict;
use warnings;

a(10);
a(0);
a(20);

sub a {
    my $num = shift;
    my $x = $num * 100 if $num;
    print $x, $/;
}
# produces
# 1000
# Use of uninitialized value $x in print at ./myif line 13.
# 2000
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Real state variables are now available with the state declarator:

sub c {
    state $count = 10;
    return $count++;
}

print c(), $/; # 10
print c(), $/; # 11
print c(), $/; # 12
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• another intruder with the mooring in the heart of the Perl

5.10 throws a warning for that type of construct

No it doesn't. I seem to recall that blead did, at some point, but it appears to no longer do so.

I constantly wonder why it is not an error at all. Whatever its behaviour could be in an actual implementation, I can see no logical nor intuitive semantics that could be attached to the construct. It has a meaning to do something upon a condition. How can have a meaning for a thingie to exist or not, upon a condition? Point is, declaration happens at compile time, condition is verified at runtime. "Ideally" one could expect that in statements following

my $var=whatever if condition;
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$var would be the lexical one just declared if condition is true, and something else (as if the declaration just were not there) otherwise: clearly, any way you look at it, it's unsatisfactory.

Not so much a bug as an unmapped and(possibly) unmappable area of the Perl landscape. What is the sensible behaviour? Personally I think an error is appropriate.

---

DWIM is Perl's answer to Gödel

In my opinion, the sensible behavior, and what most people expect when they use it, is that it would operate the same as if the my($x) was on a separate line. If that isn't possible, it should throw an error.