That's overly complicated. Just include the timestamp the link expires on plus a hash code based on the timestamp, like follows: 
use strict;
use warnings;

my ($url, $key, $exp, $hash, $link);
$url = 'http://www.domain.com/folder/page.html';
$key = 'm239VdSn';

$exp = time() + 60*60*24;
$hash = crypt(substr($exp,-8,8),$key);
$hash = substr($hash, 2);
$hash =~ s/[^a-zA-Z0-9]//g; $hash = uc($hash);

$link = "$url?$exp-$hash";
print $link;

[download]
Note that crypt only works on the first 8 characters of the given text, so you want to feed it the last 8 characters of the timestamp. The modifications to the resulting hash were to make the link look prettier.

EDIT: Added a line to remove the first two characters of the hash, which when using crypt, contain the salt.

In reply to Re: Ideas for implementing download links that expire, via e-mails by TedPride
in thread Ideas for implementing download links that expire, via e-mails by ghenry

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