By "replace the enciphered file with the original" I assume you mean decrypt? There are many modules in the Crypt:: namespace on CPAN, one of which should be suitable for your needs. The perldoc for Crypt::Blowfish provides a very straightforward example of encrypting and decrypting:

         use Crypt::Blowfish;
         my $cipher = new Crypt::Blowfish $key;
         my $ciphertext = $cipher->encrypt($plaintext);
         my $plaintext  = $cipher->decrypt($ciphertext);
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