You can do it slightly differently by putting the regex modifier(s) inside the regex:

use strict;
use warnings;
my $opts = "io";
my $regex = "test";
while (<DATA>) {
    my $line = $_;
    print $line if $line =~ /(?$opts)\Q$regex\E/;
}
__DATA__
untested
unknown
[download]

Note that this reveals a slight problem - I can't find your 'o' modifier in the docs. Note also that I have wrapped the regex itself in \Q\E to prevent problems if it contains metacharacters.

Regards,

John Davies