and why's that? It's because lexical variables cannot access the symbol table.

No, its because they're initialized at runtime and stored in a pad/Scratchpads, which are attached to subroutines ... http://search.cpan.org/dist/illguts/

As for my $a getting optimized away, I disagree with that because I just don't see how it can be optimized away when it's uninitialized in the subroutine. Here's a little test that I ran:

Your example is different.

In your example, $a is initialized after the call to mt2.

And you never remove a reference to $a, so it never gets optimized away.

Here is what you intended :)

$a is defined

#!/usr/bin/perl --
use strict;
use warnings;
print ASDF(19), "\n";
BEGIN {
    my $a = 22;
    sub ASDF {
        my $b = $_[0] + 1;
        $a + $b;
    }
}
[download]

perl.exe -d pm.919064.1.pl
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(pm.919064.1.pl:4):       print ASDF(19), "\n";
main::(pm.919064.1.pl:5):       BEGIN {
  DB<1> s
main::ASDF(pm.919064.1.pl:8):           my $b = $_[0] + 1;
  DB<1> x $a
0  22
  DB<2> s
main::ASDF(pm.919064.1.pl:9):           $a + $b;
  DB<2> s
42
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<2> q
[download]

The "problem" of $a being undefined, is because it is optimized away -- really because the sub which we're stepping into (ASDF) doesn't refer to it

#!/usr/bin/perl --
use strict;
use warnings;
print ASDF(19), "\n";
BEGIN {
    my $a = 22;
    sub ASDF {
        my $b = $_[0] + 1;
    }
}
[download]

perl.exe -d pm.919064.2.pl
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(pm.919064.2.pl:4):       print ASDF(19), "\n";
main::(pm.919064.2.pl:5):       BEGIN {
  DB<1> s
main::ASDF(pm.919064.2.pl:8):           my $b = $_[0] + 1;
  DB<1> x $a
0  undef
  DB<2> s
20
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<2> q
[download]

Here, a different sub refers to $a, so when we step into ASDF, $a is still undefined -- it is not attached to ASDF's Scratchpad

#!/usr/bin/perl --
use strict;
use warnings;
print ASDF(19), "\n";
BEGIN {
    my $a = 22;
    $a += 0;
    sub ASDF {
        my $b = $_[0] + 1;
    }
    sub DADA  { $a }
}
[download]

perl.exe -d pm.919064.4.pl
Default die handler restored.

Loading DB routines from perl5db.pl version 1.07
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(pm.919064.4.pl:4):       print ASDF(19), "\n";
main::(pm.919064.4.pl:5):       BEGIN {
  DB<1> s
main::ASDF(pm.919064.4.pl:9):           my $b = $_[0] + 1;
  DB<1> x $a
0  undef
  DB<2> s
20
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<2> q
[download]

You should consider not answering or prefacing your guesses (see To Answer, Or Not To Answer....), because there won't always be someone there to answer when Duty Calls