Subroutines in a regex?

heidkamp has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (regular expressions)

I want to replace text with the results of a subroutine, ie

 
s/SUM(\d*)/sum($1)/g

sub sum()
[download]

using (?{CODE}) is 0 width (no replacement), and I can't get it to work in the replacement section anyway.
Any ideas?

Originally posted as a Categorized Question.