my $re = do {
    local $" = '|';
    qr/^@{[map {quotemeta} @list]}$/;
};

##</code><code>##

my @sets;
for (@list) {
    my $str = $_;
    for (0..(length-1)) {
        $sets[$_] .= substr $str, $_, 1;
    }
}
my $re = qr/^${\join '', map {"[$_]"} @sets}$/;

##</code><code>##

{
    my %test_hash;
    @test_hash{@list} = ();
    sub efficient_test { exists $test_hash{+shift}; }
}