That can be done with qr// as well though.

my $rx = qr/^@{[shift @ARGV]}$/;
my @match = grep /$rx/, <>;
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qr// also is much less likely to be used mistakenly than /o and much easier to spot when it is. /o should be considered a relic from pre-qr// times that has few if any uses anymore.

Makeshifts last the longest.