my @Found = ();

while (my $line = <DATA1>)
{
chomp $line1;

while (my $line2 = <DATA2>)
{
chomp $line2;

if ("$line1" eq "$line2") {}
else { push @Found, $line2 }
}
}

open (DATA3, "data3") or die;
{
print @Found;
}