That reverse sort isn't necessary (unless I'm missing something). Instead, sort it in the reverse order in the first place:

my @stopwords= qw/ car pet carpet /;
my $stopmatch= join '|' => 
  map { '\b' . $_[1] . '\b' }
  sort { $b->[0] <=> $a->[0] } ## No reverse here
  map { [length $_, $_] } @stopwords;
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I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated