I'm not absolutely sure of the reason for this but if you change your program to use Digest::MD5 in the functional style then you will find it returns the same digest each time - i.e.:

use Digest::MD5 qw(md5_hex);
#...
my $cs = md5_hex($foo);
[download]

I'm guessing its something to do with the way that the module keeps state in OO usage, but I can't be sure without looking at the code.

/J\