A second approach to the one mentioned above is to exploit the power of the /e modifier, which turns the right-hand side of the substition into a first-class expression. Then you could do something like:

  s/([a-m])([n-z])/$1 . myfunction($1, $2) . $2/eg;

  sub myfunction {
    my ($pre, $post) = @_;
    # we could do something smart with the pre- and post-
    # strings, but instead we'll just return a 5.
    return 5;
  }
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