It seems like you want to rewrite the configuration file of your mail program so that it takes the parameters of the person who is logged on on your system through the Perl script.

Well, this is certainly an easy task to realize in Perl, sonce Perl is so good at text file operations...
But it makes it a little funny that you expect a Monk to guess what to write for you just by giving $u{'user'} and $u{'pass'} for information... Everybody knows that Monks do like to communicate using telepathy, but for this problem I'm not sure somebody will help you.

Monks are certainly willing to help you even if you want the solution "cooked for you" so that you just have to "cut-and-paste", but in this case you will have to at least give the name of the configuration file and show us what are the lines that you want to change inside your Perl script!
Good luck!