To answer your question literally, yes:

{
  my $count = 0;
  s/(\d+_([a-z]+))/++$count > 2 ? $1 : $2/eg;
}
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Here the $count variable is counting instances of matches. When it gets past two, the original string is replaced with itself, otherwise the desired change is made. The /e modifier says that the right side is Perl code to be executed rather than simply a double-quoted string.

-- Randal L. Schwartz, Perl hacker