There are some more complicated cases there. If you are interested, try with yahoo email.

The web page that yahoo email used to authenticate you, would send out your password encryted for security, to make it even complex, they send you a challenge, which is a string, and your password would be encrypted together with the challenge.

By doing this, they are trying to make sure nobody can pirate your session, at least the solution to pirate your session is much less obvious.

You can study yahoo's authentication page, and try to do the same encryption with Perl, which obviously has no problem with Perl ;-).

It is MD5.

You can make up specific programs, but it would be quite difficult to make up a generic program to deal with all web sites, without the ability to execute scripts in other languages.

However, it is still fun to play with yahoo as your next step.