#!/usr/bin/perl -w
use strict;
print scalar localtime(),"\n";
slow_once(5);
slow_once();

{
    my $expensive_thingy;
    sub slow_once {
        unless (defined $expensive_thingy) {
            # make expensive thingy:
            sleep 5;
            $expensive_thingy = shift;
        }
        print scalar localtime()," :$expensive_thingy:\n";
    }
}
__END__

OUTPUT:
Sat Nov  3 15:55:31 2001
Sat Nov  3 15:55:36 2001 :5:
Sat Nov  3 15:55:36 2001 :5: