Because it's unnecessary. Why would you want to perform operations in your code that weren't necessary? Would you write a hello world program like this:

use strict;
use warnings;
use 5.012;


if ("hello world" =~ /hello/) {
    print 'hello';
}

if ("hello world" =~ /world/) {
    print " world\n";
}

--output:--
hello world
[download]

That program unnecessarily uses program resources, and therefore it is bad programming style.