I'll accept that reasoning. :)

Here's a version of your 'add elipses' regexp that shifts the code evaluation to the left side of the s/// operator by using (?{...}). Not as clear as your solution, but another way to do it. I'm always looking for entertaining ways to use (?{...}). ...your approach is more appropriate for actual use though. Keep mine in the cage.

$string =~ s/^(.{1,20})(?{(length($_)>pos)?'...':''}).*$/$1$^R/;
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Just for fun... ;)