How do I use a variable as a regex modifier in Perl?

Question

I\'m writing an abstraction function that will ask the user a given question and validate the answer based on a given regular expression. The question is repeated until the

Answer 1

You might also like the qr operator which quotes its STRING as a regular expression.

my $rex = qr/(?$mod)$pattern/;
$isValid =  =~ $rex;