Thanks everyone. I eventually got it working... I ended up using the perl implementation of md5 from cpan The instructions given on cpan tell you to use:

use Digest::MD5;
$ctx = Digest::MD5->new;
[download]

but that would not work, and a bit of trial and error brought to me this:

use MD5;
$ctx = Digest::Perl::MD5->new;
[download]

which now works well. I don't know why their instructions didn't work down my end, but if I got it wrong, how and why? And if it is wrong on cpan, does the author need telling?