Well, without reading the explanation for the warning, I would expect ()(BBB)() or (AAA)(BBB)(AAA) or just ()()() like the case of no parameters.

It seems that sub f (the second one) compiles in runtime, and internal $a is not asigned the first time it is called, but when called from g, it glues the first value it receives.

Let me show an improved example:

#!perl
use v5.10;
use strict;
use warnings;

my $a = shift;

sub f {
  say "global f ($a)";
}
f();

sub g {
  my ($a) = @_;
  say "global g ($a)";
  sub f {
    say "local f ($a)"; # line 17
  }
  f();
}

f();
g(shift);
say "main1 $a";
$a = shift;
say "main2 $a";
f();
g(shift);
f();

__END__

C:\Temp>localsub.pl AAA BBB CCC DDD
Variable "$a" will not stay shared at C:\Temp\localsub.pl line 17.
Subroutine f redefined at C:\Temp\localsub.pl line 16.
Use of uninitialized value $a in concatenation (.) or string at C:\Tem
+p\localsub.pl line 17.
local f ()
Use of uninitialized value $a in concatenation (.) or string at C:\Tem
+p\localsub.pl line 17.
local f ()
global g (BBB)
local f (BBB)
main1 AAA
main2 CCC
local f (BBB)
global g (DDD)
local f (BBB)
local f (BBB)
[download]

As you can see, the internal variable is empty (uninitialized) the first two times f is called, but as soon as it is defined, it keeps that value forever. Weird...