The way I'd write this is thus:

#!/usr/bin/perl -w
use strict;
use Net::SSH qw(ssh);

use constant USER => 'root';
use constant CMD => '/usr/local/scripts/temp.sh';
 
for my $host (qw(
    192.168.15.2
    192.168.16.2
    192.168.17.2
    192.168.18.2
)) {
    (ssh(USER . '@' . $host, CMD) != 256)
        and last;
    warn "No response from $host\n";
}
[download]

Of note is that your original code had == '256' comparisons, mixing a numerical operator with a string operand. Why? Either use == 256 (numerically compare against a number) or eq '256' (string comparison against a string). As you really mean the number 256 here, the former is the right choice, even though what you had works too. It does incur a pointless string-to-number conversion, but that's beside the point: you need to be aware of the difference because one day it will bite you if you treat them as interchangeable.

Makeshifts last the longest.