http://perldoc.perl.org/perlsyn.html:

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 very much, ysth for the explanation.

My dense skull isn't penetrating this issue. The original poster wrote ...

Update: No, the original poster did not write what I claimed. The first response did.

my $x;  # one statement
$x = $y if {condition;}   # two statement
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the perlsyn you quoted is different

 my $x if ... 
presuming
 my $x = $y if {condition;}
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in his case. These seem to be different. You are holding up the perlsyn snip as an example says the two cases are synonyms. Why?

In my testing of this, it works (perhaps version dependent?):

sunorccws04 ~$ cat tpl ; ./tpl
#!/usr/bin/perl
sub showme;

foreach $y ( 1..10) {
    $foo = $y;
    print showme(), "is the x value\n";
}

sub showme {
        my $x;
        $x = $y if ($foo %2); # might as well mix up some T/F
}
1 is the x value
0 is the x value
3 is the x value
0 is the x value
5 is the x value
0 is the x value
7 is the x value
0 is the x value
9 is the x value
0 is the x value
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getting strict "breaks" the behavior,

sunorccws04 ~$ cat tpl ; ./tpl
#!/usr/bin/perl
use strict;
my ($foo, $y);
sub showme;

foreach $y ( 1..10) {
    $foo = $y;
    print showme(), " is the x value\n";
}

sub showme {
        my $x;
        $x = $y if ($foo %2); # might as well mix up some T/F
}
 is the x value
0 is the x value
 is the x value
0 is the x value
 is the x value
0 is the x value
 is the x value
0 is the x value
 is the x value
0 is the x value
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predefining x as in  my $x = 0;, doesn't change the results. Changing the declaration of $y, $foo from my to our causes the code to behave like the non-strict version. But in all cases the results are consistant and predictable.

??

Be Appropriate && Follow Your Curiosity

My dense skull isn't penetrating this issue. The original poster wrote

my $x;  # one statement
$x = $y if {condition;}   # two statement
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Both when I posted and now, the OP reads: my $x = $y if ( <some condition> ); Are you looking at someone else's reply?

With the my $x separate and unconditional, the warning in perlsyn doesn't apply.

The output from your strict version doesn't make sense to me, offhand.

my $x;
$x = $y if ( <some condition> );
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Another possibility, if you still wanted it on one line, would be:

sub sub1 {
    #...
    my $x = ( <some condition> ) && $y;
    #...
}
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This way, if the condition ended up false, you'd wind up with some false value (although not necessarily undef) in your variable. You could do my $x = ( <some condition> ) ? $y : undef; if you need $x to default to undef, but I'd say Siddartha's solution looks tidier for that case.

I like your suggestion to use the ternary operator. In fact, it makes more sense than the high-precedence logical operator approach you outline above. The logical approach relies on short-circuit effects to work, and may not be obvious to future maintainers. The ternary operator is being used in its normal mode.

Another issue with the logical approach is that when the condition is false, the value of $x becomes whatever scalar the condition evaluates to. This could lead to unexpected behavior where the condition may evaluate to an otherwise legal value for $x.

my $y = 0;

# ... Much code;

my @list = ();

# ... Much code

my $x = @list && $y;
# if $x is 0, did it come from @list or $y?
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If the OP wants to do a conditional assignment and declaration on one line, the following is, IMHO, unambiguously the way to go.

sub sub1 {
    # ...
    my $x = <some condition> ? $y : undef;
    # ...
}
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TGI says moo

It should, yes. But don't expect it to in any 5.8.x release. The maint pumpking is very cautious about adding new warnings, and for a while there were a number of people advocating

sub foo {
   my $x if 0;
   ...
}
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to create a "static" variable, rather than using a closure

{
   my $x;
   sub foo {
      ...
   }
}
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sub mehtod($)
{
    my $y = @_;
    my $x = "";  # or    
    my $x = 0;   # then    
    if(/expression/)
    {
      $x = $y;
    }
}
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