$ perl -Mstrict -Mwarnings -E '
    my @lexicon = qw{own known};
    my $string = q{unknown};
    my $alt = join q{|} => sort { length $b <=> length $a } @lexicon;
    my $re = qr{ ( (?> $alt ) ) $ }x;
    $string =~ $re;
    say $1;
'
known