The absence of a stupid typo! Sorry, the regex should have been $original_text=~ s/(?<=\d)mp\b//g;.

By the way I strongly recommend that you add use strict; use warnings; to any script you write. You then need to declare variables using my, but many problems get found before they bite hard.

You should also browse the following documentation, probably in the order given: perlretut, perlre and perlreref.