m// and s/// are two completely different operators. Do not try to shoehorn them into a single expression like that - you will just be creating less maintainable code. The best solution is not always the one with fewest keystrokes.

if ($some_condition) {
  ($foo) = ($bar =~ m/capturing_pattern/);
else {
  ($foo = $bar) =~ s/pattern/replacement/;
}
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That should hopefully be unambiguous and more maintainable than the original proposal.