#!/usr/bin/perl
use warnings;
use strict;


foreach my $num (reverse (999 ... 999*999)) {  # I skipped the 'product of two 3 digit integers' part: see LanX below

    if ($num eq reverse $num){
        print "largest palindrome is: $num\n";
        exit;
    }
}

# or just:
perl -we "for(reverse 999 .. 999*999){($_ eq reverse $_ )?(print qq(largest palindrome $_)and exit 0 ): 1}"

largest palindrome 997799