I don't know exactly what you meant to match with the following two REs, but guessing from the others, I think you are missing something:

$post =~ s!\Q:-\\E!<img src="./emoticons/what.gif">!gio;
$post =~ s!\Q:\\E!<img src="./emoticons/what.gif">!gio;
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All your other REs end with \E (presumably to close the \Q) but those two do not, they end with just the letter E. You may be missing a \ in there.

And benchmarking with the missing \ does show that the should_be_faster indeed is.

Array loaded with 84 posts.
Begin benchmark.
                   Rate should_be_slower should_be_faster
should_be_slower 1331/s               --             -14%
should_be_faster 1552/s              17%               --
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