use warnings;

$main::look{mom} = "works";
my $var = "It works!";
if ( $var =~ /$main::look{mom}/ ) {
    print "Guess what!  It matched!\n";
}
[download]

Variable interpolation occurs before the regexp engine revs up and starts its work. See perlop under the "Gory details of parsing quoted constructs" section. The variable interpolation stage gets performed a couple passes before the regexp engine kicks in.

However, it may still be necessary to use \Q and \E, if the content of your interpolated variable could, itself, be misinterpreted as metacharacters.

Here's contrived example of a ? and a . being seen as metacharacters when they were probably intended to be seen as literal text:

use strict;
use warnings;

my $test_interpolation = "Old McDonald had a farm?.";

if ( "Old McDonald had a fart " =~ m/$test_interpolation/ ) {
    print "Ewwwww!\n";
}
__OUTPUT__
Ewwwww!
[download]

Wrapping a \Q and \E around the variable interpolation inside the regexp would cure this problem, as would using the quotemeta function.