As far as I can tell, those two regexes are identical: one as a plain match, and one as a substitution.
I'm guessing somebody is first testing to see if anything is found, and if it does, replace it.
Don't do that. Just plain s/// is enough, it won't do anything if nothing is found, so it's safe.

And if it happens to be the case that the existence of a match is used to conditionally control more than just a substitution, the  s/// operator returns the number of substitutions done, or false if there is no match, so it works just fine as a conditional expression. See s/PATTERN/REPLACEMENT/msixpogce in perlop.

>perl -wMstrict -le
"my $str = 'foo';
 ;;
 if ($str =~ s{ foo }{bar}xms) {
   print qq{substituted with $str};
   }
"
substituted with bar
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