The main (and very bold) argument for the substitution being an side-effect operator is the need to know whether it succeeded. You often need to do this:

s/^($timestamp_re)/decode($1)/e or die "Oh wait, looks like your log i
+s garbled!\n";
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...or this:

s/11 Aug/10 Aug/g and ++$count;
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How will you know if your substitution function found anything to substitute?