As Transient said, it depends on your criteria. If it's the underscore-period combo you need taken care of, then gellyfish's solution should suffice. If it's just the last underscore no matter what, then a greedy regex like this could work:

  my $txt = "This_has_under_scores.mp3";
  $txt =~ s/(.+)_/$1/;
  print $txt; #  "This_has_underscores.mp3";
[download]

Update: ... or non-greedy from the other end:

  $text =~ s/_(.*?)$/$1/;
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