There's a couple ways you could do it. Here's one, but there can't be a '.' or a '|' in the file name.

$string=~ /\s*([^.|]+)\.mp3/; 
print "$1\n";
[download]

Hope this helps,
Rich

update
Use:

$string=~/\s*([^|]+)\.mp3/;
[download]

The period wasn't needed.