my $xor = $file1contents ^ $file2contents;

my $max = 0;
my $max_str;
my $pos;
while ($xor =~ /(\0+)/g) {
	my $len = length $1;
	if ($len > $max) {
		$max = $len;
		$max_str = substr $file1contents, $-[0], $len;
		$pos = $-[0];
	}
	#print "matched $-[0] ", substr $file1contents, $-[0], $+[0] - $-[0];
}

print "at pos $pos max string is $max_str";