$val = '123';
while (<DATA>) {
    s/\$(\w+)/$$1/g;
    print;
}

__DATA__
test_string_$val

##</code><code>##

use strict;
use warnings;

my %subs = (val => '123');
while (<DATA>) {
    s/\$(\w+)/$subs{$1}/g;
    print;
}

__DATA__
test_string_$val