You can't, and it doesn't make much sense to do so. Maybe you want something like this:

    my $matched_pattern;
    if (/$regex/) {
        $matched_pattern = $regex;
    };
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If you're really bent on filling $1 with an arbitrary string, you can do so by doing:

    my $regex = qr/foo/;
    $regex =~ /^(.+)$/ms; # fill $1 with $regex
    print $1;
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This does not make sense to me, so most likely I misunderstood your question. If you want something else, please tell us the output you want.