if ($string =~ /${\(TEST)}/) {

##</code><code>##

use constant PATTERN => qr/def/;
if ($string =~ PATTERN) { ... }

##</code><code>##

use constant PATTERN => 'def';
my $regex = qr/${\(PATTERN)}/; #options such as /m can go here.
if ($string =~ regex) { ... }