#!/usr/bin/perl

open MY_PID, "> /tmp/do_stuff.pid"
  or die "can't write to /tmp/do_stuff.pid: $!";
print MY_PID $$;
close MY_PID;

# rest of code

##</code><code>##

#!/usr/bin/perl

open PID, "/tmp/do_stuff.pid"
  or spawn();
chomp(my $pid = <PID>);
close PID;

kill -HUP => $pid or spawn();


sub spawn {
  fork and exit;
  exec "/usr/bin/perl", "my_prog.pl";
}