use strict;
use warnings;
use Devel::Peek;

$\=$/;

sub foo {
    my $bar = @_ ? shift : 1;
    print "in foo(): "; Dump($bar);
    sub bar {
        my $foo = shift;
        print "in bar(): "; Dump($bar);
        return $foo * (++$bar);
    };
    my $result = bar($bar);
    $result;
}
print "calling foo(1): 1 * 2 = " . foo(1);
print "calling foo():  1 * 2 = " . foo();
print "calling foo():  1 * 2 = " . foo();
print "calling bar(5): 3 * 5 = " . bar(5);
print "calling foo(3): 3 * 4 = " . foo(3);
__END__
Variable "$bar" will not stay shared at bar.pl line 17.
in foo(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 1
in bar(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 2
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 1
calling foo(1): 1 * 2 = 2
in foo(): 
SV = IV(0x88d0e0c) at 0x88b5720
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 1
in bar(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 2
calling foo():  1 * 2 = 3
in foo(): 
SV = IV(0x88d0e0c) at 0x88b5720
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 1
in bar(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3
calling foo():  1 * 2 = 4
in bar(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 4
calling bar(5): 3 * 5 = 25
in foo(): 
SV = IV(0x88d0e0c) at 0x88b5720
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 3
in bar(): 
SV = IV(0x88d0b1c) at 0x88b57a4
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,IOK,pIOK)
  IV = 5
calling foo(3): 3 * 4 = 18
[download]

Note how the addresses reported as "at 0x..." after the line "in bar():" are the same throughout, while the location in foo() changes after the second invocation. This might be a bug or a glitch in the perl engine, but at least it is a documented one (through diagnostics and warnings). AFAICS it is a compile-time/run-time issue.

There are two ways to circumvent this. One is the 'static variable hack' which will still emit the warning:

use strict;
use warnings;

sub foo {
    my $bar if 0; # <--- HACK!
    $bar = @_ ? shift : 1;
    sub bar {
        my $foo = shift;
        return $foo * (++$bar);
    }
    my $result = bar($bar);
    $result;
}
warn "calling foo(1): 1 * 2 = " . foo(1) . "\n";
warn "calling foo():  1 * 2 = " . foo()  . "\n";
warn "calling foo():  1 * 2 = " . foo()  . "\n";
warn "calling bar(5): 3 * 5 = " . bar(5) . "\n";
warn "calling foo(3): 3 * 4 = " . foo(3) . "\n";
__END__
Variable "$bar" will not stay shared at bar.pl line 17.
calling foo(1): 1 * 2 = 2
calling foo():  1 * 2 = 2
calling foo():  1 * 2 = 2
calling bar(5): 3 * 5 = 15
calling foo(3): 3 * 4 = 12
[download]

This hack prevents the my $bar variable to be cleared, since the optimizer weeds out that code because of the constant comparison if 0;, but the compiler is glad since it has seen a lexical declaration.

The other way is making up an anonymous subroutine and assigning that to a typeglob:

use strict;
use warnings;

sub foo {
    my $bar if 0;
    $bar = @_ ? shift : 1;
    unless (*bar{CODE}) {
        my $bar_sub = sub {
            my $foo = shift;
            return $foo * (++$bar);
        };
        *bar = $bar_sub;
    }   
    my $result = bar($bar);
    $result;
}
warn "calling foo(1): 1 * 2 = " . foo(1) . "\n";
warn "calling foo():  1 * 2 = " . foo()  . "\n";
warn "calling foo():  1 * 2 = " . foo()  . "\n";
warn "calling bar(5): 3 * 5 = " . bar(5) . "\n";
warn "calling foo(3): 3 * 4 = " . foo(3) . "\n";
__END__
calling foo(1): 1 * 2 = 2
calling foo():  1 * 2 = 2
calling foo():  1 * 2 = 2
calling bar(5): 3 * 5 = 15
calling foo(3): 3 * 4 = 12
[download]