I'm not sure what you want to do exactly, but if you want to just substitute the hash key into the corresponding hash value, this will work.

# here's the hash
%s = (   "substring1" => 'string1',
         "substring2" => 'string2',
         "substring3" => 'string3'
     );

# get the current keys
@keys = keys(%s);

# go through each key and sway the key/value
foreach $key (@keys) {
    $s{$key} = $key;

    # print out the swapped hash value
    print "$s{$key}\n";
}

Output
----------------------------------
substring1
substring2
substring3
[download]

Not sure if this was what you were looking for or if there is a better way to do it with regexps, but maybe this will help a little. -Eric