#!/usr/bin/perl -w

use strict;

my $opts = "io";
my $regex = "test";

my $file = "./testfile";
open(my $fd, '<', "$file") or die "$!";
my @ary = <$fd>;
close $fd;
for my $line (@ary) {
        print $line if ($line =~ /$regex/$opts);
}

> ./test.pl test.txt 
Scalar found where operator expected at ./test.pl line 13, near "/$regex/$opts"
	(Missing operator before $opts?)
syntax error at ./test.pl line 13, near "/$regex/$opts"