length is an extremely cheap operation, so an ST is superfluous clutter here. The full-words-only effect does not require an extra map either - just stick the assertions around the final regex.

my $stopmatch =
    join '|',
    sort { length $b <=> length $a }
    qw(car pet carpet);

$stopmatch = qr/\b(?:$stopmatch)\b/;
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Note that sorting on length is only correct as long as you don't have regex quantifier expressions in the "words" - if you do, all bets are off. At any rate, if you're going to this effort, you should probably be using Regex::PreSuf.

Makeshifts last the longest.