$ cat regex_in_str.pl
#!/usr/bin/perl
use strict;
use warnings;

my $r = "ab+c*d";
my $rRegex = qr/$r/;

for my $t (qw( abcd aabbbd acccd dbca )) {
	if ($t =~ $rRegex) {
		print "MATCH: $t\n";
	}
}

$ perl regex_in_str.pl
MATCH: abcd
MATCH: aabbbd

$