Consider this litte program:

#!/usr/bin/perl
use strict;
use warnings;

our $foo;

sub do_something {
  $foo++;
  print "$foo\n";
}

our $running;
if (!$running) {
  $foo = 0;
  $running = 1;
  $SIG{INT} = sub {do $0};
  for (;;) {
    sleep (1);
    do_something();
  }
}
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This will output increasing numbers.
Now change its source code while it is running, for instance change "$foo++" to "$foo+=5". Then send the program a SIGINT by pressing ^C (don't know if this works on Windows). You will get a warning about a redefined subroutine, but then the program continues running with its old variables and a new do_something subroutine.
I experienced that this is particularly useful for programs like IRC bots to change the code without having to stop the programs.
If you put the subroutine into a module, you will probably be able to leave out the $running stuff and make everything simpler.