Seriously, if you are wanting to learn more about encryption I would highly recommend picking up Applied Cryptography: Protocols, Algorithms, and Source Code in C by Bruce Schneier at some point...

Yes, I know...it's not a perl book but it is an excellent resource on cryptography programming as evidenced by the well used copy I keep borrowing from DrManhattan

For your specific application, are you wanting to use symmetric encryption...say, to store a file away from prying eyes. Or, do you need something asymmetric like PGP to send an encrypted file...i.e. an e-mail to someone so they can decrypt it?

If you are wanting symmetric encryption of a file, then Crypt::CBC is a breeze...

To Encrypt using Blowfish...

#! /usr/bin/perl
 use Crypt::CBC;
  $cipher = Crypt::CBC->new( {'key'             => 'my secret key',
                              'cipher'          => 'Blowfish',
                              'iv'              => '$KJh#(}q',
                              'regenerate_key'  => 0,   # default true
                              'padding'         => 'space',
                              'prepend_iv'      => 0
                           });

$cipher->start('encrypting');
open(F,"./plain_file");
open(STDOUT,">crypt_file");
  while (read(F,$buffer,1024)) {
      print $cipher->crypt($buffer);
  }
close STDOUT;
[download]

$cipher->start('decrypting');
open(F,"./crypt_file");
  while (read(F,$buffer,1024)) {
      print $cipher->crypt($buffer);
  }
[download]

That should get you started