G'day victorz22,

When you simply want to test for a substring, use index: you'll find it's measurably faster than a regex. [You can use Benchmark to measure it.]

Here's the technique you need:

#!/usr/bin/env perl

use strict;
use warnings;

while (<DATA>) {
    print index($_, 'SUBSTR') > -1 ? "REPLACEMENT\n" : $_;
}

__DATA__
path/to/some/file
path/to/some/other/file
path/to/SUBTSTRING/file #replace entire line if SUBSTRING is found
path/to/file
[download]

Output:

path/to/some/file
path/to/some/other/file
REPLACEMENT
path/to/file
[download]