This looks surprisingly like another question asked here yesterday. Homework assignment?

As for the questions ...

1. To "undef a variable", you simply undef $foo;. The rest of your first question indicates that you don't really understand what you're asking. Think about it further - what exactly do you want to do? If you don't know, then nothing I say can help you.
2. Swapping a character for another character is called transliteration. Perl uses an operator called tr. Click on the hyperlink for more information.

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Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.