i still don't understand, why you're usiińg substr() at all - instead of a regex

Speed

$str =~ s/_(\.mp3)$/$1/g;
# Or Much faster with the same result:
substr($str, -5, 1, "") if(substr($str, -5, 1) eq '_');
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If you do not need the power of metacharacters which a regex offers, don't use a regex. Using a regex for this opperation is like using a sledge hammer to pound in a small nail.