You've got the right idea; you can edit the script in notepad or any other text editor, and if Crypt::Blowfish_PP is designed to be a drop-in replacement for Crypt::Blowfish, you should be able to make the changes you describe. Whether it will work perfectly or not will require you to try it and test it.

There's no real way to make the key invisible in the program, since the program needs the key. You can take various measures to obfuscate it, but they can all be figured out by anybody with a little perl knowledge. You can protect it with OS-level protection, like file permissions controlling who can see and run the script.

Hello, Recently I got a PERL program written that will encrypt file using Blowfish.

It's Perl, not PERL.

The file cryptdata.pl has the following line: use Crypt::Blowfish;. [...] can I change the line to use Crypt::Blowfish_PP; using NotePad?

Probably. Try it.

can I change the line:
my $proc = new Crypt::Blowfish $key_valid;
to
my $proc = new Crypt::BlowfishPP $key_valid;

That should be
my $proc = new Crypt::Blowfish_PP $key_valid;

The source file has: my $key = "My key" Now this key is in pure text format. How can I encrypt this key itself?

You can't. Then you'd need another plaintext key to decrypt the key.

What you do is control access to the .pl using your OS's permission system. Actually, Better yet, move the key to a seperate file and control access to both the .pl and the key file. That way, it's easier to use revisioning systems on the .pl, to make copies of the .pl, etc.

If this is not possible, then can I compile this cryptdata.pl file into an executable?

That won't help. It'll still be just as visible to the trained eye.

What is difference between Blowfish and Blowfish_PP? Will the output differ if I change Blowfish to Blowfish_PP?

Hopefully, speed is the only difference. That's why I suggested you simply try to use _PP.

If you use PGP or GPG then the key used to encrypt it can't be used to decipher it. In that way it would be more secure. Just a though.

Since Perl comes from a UNIX background, it is assumed that UNIX tools including a compiler are available to you. Windows users can also install these tools or make use of pre-packaged distributions.

My recommendation if you are starting out with Perl is to install the ActiveState distribution since this is well known and has good support for downloaded packages. You will need to learn how to use the PPM (Perl package manager) tool that is used to find and install modules.

I have added the University of Winnepeg repository to my PPM configuration since this has better support for the Crypt:: modules.