Consider using

perl -MCPAN -e "shell"
> install Digest::MD5
[download]

to install modules. It will make your life easier (if you aren't already doing that, that is).

Digest::MD5 is the go-forward standard module to be included with your perl distribution (if 5.7 development is any indication). If your program is already written you should be able to install the plain old MD5 module as well, it does show up on search.cpan.org search for "md5".