Crypt::Blowfish_PP only provides means of encrypting and decrypting 8-byte blocks. Being able to encrypt and decrypt blocks is just part of "encryption".

• Were you just passing random length strings to encrypt? You need padding.
• Were you planning on encrypting every block with the same key? You need chaining.
• Were you planning on encrypting every conversation with the same key? You need salting.

Crypt::CBC is a facility to provide padding, chaining and salting.

That still leaves

• key management, (How do you give the key to both hosts? How does each host protect the key? How do you know the protection works? What happens when the protection fails? Until when is the key valid? What happens if the key is compromised? etc.)
• algorithm management, (How do you know Blowfish is still secure? What happens when Blowfish is not secure? How do you know the implementation is secure? What happens when the implementation is no longer secure? etc.),
• ...

And that's just to protect the data in transit.