Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Well I ran into this construct that doesn't do what I expect. Is Perl's behavior justified? If so, please help me understand. 
#!/local/bin/perl -w
use warnings;
use strict;

# Runs the sub reference in arg #1
sub Doit
{
&{$_[0]};
}

# Start of a big block - don't want to pollute
{
my $x = 777;
my $y = 222;

print "Initialized, what's the values?";
print " x = $x\n";
print " y = $y\n\n";

sub mysub
    {
    print "  Start of subroutine\n";
    print "    what's x? (forget y for now)\n";
    print "    x = $x\n\n";
    print "    Going to child block\n";
    print "     let's inspect the variables again:\n";
        {
        Doit( 
            sub { 
            print "x = $x\n\n"; 
            print "      y = $y\n\n"; } );
        }
    }
}
# Big Block 
mysub();

[download]
---------------------- output: 
>perl perlbug.pl
Initialized, what's the values? x = 777
 y = 222

  Start of subroutine
    what's x? (forget y for now)
    x = 777

    Going to child block
     let's inspect the variables again:
x = 777

Use of uninitialized value in concatenation (.) or string at perlbug.pl line 31

      y =
(This behavior has been checked on Perl 5.6.1 and 5.8.8)

Replies are listed 'Best First'.
Re: Lexical Scoping and Anonymous Subroutines Passed To Functions
by jdporter (Paladin) on Jun 15, 2007 at 21:30 UTC

    Yeah, it's a known quirk. Perl doesn't think it needs to bind $y in the inner sub because it needn't in the outer sub, whereas it did have to bind $x in the outer sub. (btw - that inner bare block, around the Doit call, is superfluous; removing it doesn't change the behavior, and, I think, clarifies the situation somewhat.)

    A word spoken in Mind will reach its own level, in the objective world, by its own weight
[reply]
Re: Lexical Scoping and Anonymous Subroutines Passed To Functions
by Anonymous Monk on Jun 15, 2007 at 21:49 UTC
    I think my previous example was perhaps too involved. Here's something simpler that shows the same behavior: 
    #!/local/bin/perl -w
use warnings;
use strict;

# Start of a big block - don't want to pollute
{
my $y = 222;
my $x = 777;


print "Initialized, what's the values?";
print " x = $x\n";
print " y = $y\n\n";

sub mysub
    {
    print "  Start of subroutine\n";
    print "    what's x? (forget y for now)\n";
    print "    x = $x\n\n";
    print "    Going to child block\n";
    print "     let's inspect the variables again:\n";

    my $sr =             
        sub {
        print "y = $y\n\n";
        print "x = $x\n\n";
        };
    
    &$sr;
    }
}
# Big Block
mysub();

    [download]
[reply]
[d/l]
      This bug is fixed in perl 5.9.0

      Dave.

[reply]
Re: Lexical Scoping and Anonymous Subroutines Passed To Functions
by ambrus (Abbot) on Jun 16, 2007 at 15:30 UTC
[reply]

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