BTW, if you want to test your error-checking code, you can break the stored checksum like this:

open my $F, '+<', 'file.gz' or die $!;
seek $F, -8, 2;
my $c = getc($F);
seek $F, -8, 2;
print $F chr(ord($c) ^ 1);
[download]

Running this a second time will fix the .gz file.