Short answer : You can't !

MD5 stands for Message Digest 5
It's a one way fonction which computes a text's digest.
You can't decrypt a MD5 digest.
You usually use it to check that the digest of the value just entered is equal to the digest stored.
(usually passwd management works this way)

Use other usual encryption schemes if you want to decrypt text.
(RIJNDAEL, RSA, ELGAMAL...)

UPDATE :
I warmly recommend btrott's impressive work (Crypt::OpenPGP)
which handles several schemes
(IDEA, DES3, Blowfish, Rijndael, Rijndael192, Rijndael256, Twofish,DSA, RSA, and ElGamal)

"Only Bad Coders Code Badly In Perl" (OBC2BIP)