MD5 is ireversible. means you cant decrypt, as its only used to encrypt a string to hide it from anyones eyes, eg. entries in a passwd file.
But you might want to look at CPAN to find a module that fits most your needs. Crypt::Blowfish is quite common in use. Or even use perlmonks search function for "crypt".

Have a nice day
All decision is left to your taste