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my $command = qr/(ab)/;
Business needs might prompt the addition of longer command names, so instead of just ab one might add:
my $command = qr/(ab|abbr|abbreviate)/;
This is buggy; the regular expression will never match abbr nor abbreviate.
$ aslines ab abbr abbreviate | perl -nE 'say $1 if /(ab|abbr|abbreviate)/'
ab
ab
ab
Causes of this bug are where the alternation is thrown together at random--and never tested nor reviewed, an all too common case--or where software automatically builds the alternation and that building software is buggy. The Data::Munge Perl module by contrast takes a number pains in the list2re function to get this right.
https://metacpan.org/pod/Data::Munge
Other software or people writing software may be less enlightened about such matters. Buyer beware?