You're missing a $ in my (regex) = @_. Also, you can qr// an empty string, which will break your regex.

Simplifying and enhancing a bit more results in something like:

sub deregex {
    my $regex = shift;
    my ($ops, $pat) = $regex =~ /^\(\? ([msix-]+) : (.*) \)$/x
        or die "Unknown qr// format";

    $pat =~ s!/!\\/!g;
    $ops =~ s/-.*//;
    return "/$pat/$ops";
}
[download]

— Arien