One of the properties of hashes is that given the same input, a hashing algorithm will produce the same output. Guaranteed. Given that you're getting different output, that means that your input is different somehow. As a debugging mechanism, try printing the input value along with its hashed value. Something like:

use Digest::MD5 qw(md5_hex);
my $var = "foobar";
print join("::", $var, md5_hex($var)), "\n";
[download]

My guess is that you're using the OO interface, calling add, and not calling reset. But that's a stab in the dark.