use strict;
use warnings;
use 5.010;

my $val = 10;

sub f {
    say "in global f, \$val is: $val";
}

f(); #Demonstrates that the global definition of f is overwritten 
     #by the local definition of f (below) at compile time.  As a 
     #result, you won't see output from global f.  When the local f
     #executes instead, the global $val above is hidden by a local
     #$val in the definition of f (below).

say '=' x 20;

sub g {
    my $val = shift;
    say "in g, \$val is: $val";

    #This definition of f closes over $x in previous line.
    sub f {   #line 40
        say "in local f, \$val is: $val";  
    }

    f();
}

g('hello');
say '=' x 20;

g('goodbye');
say '=' x 20;

--output:--
Variable "$val" will not stay shared at 4perl.pl line 41.
Subroutine f redefined at 4perl.pl line 40.
Use of uninitialized value $val in concatenation (.) or string at 4perl.pl line 41.
in local f, $val is: 
====================
in g, $val is: hello
in local f, $val is: hello
====================
in g, $val is: goodbye
in local f, $val is: hello
====================