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The trailing "i" means it ignores the case.
The "g" is the global replace option.
There's a good example of "i" in Learning Perl 2nd edition.
On page 11 and the program continues on page 12.
It's really helpful for making your programs dummy proof for users.
An example of "g" can be found on page 88 of the same book.
It is my bible although I have two other books I am using.
(a programming newbie can never have enough books)
I almost forgot to mention that issuing the command "perldoc perlop"
(without the quotes) on most systems will give you
detailed information on perl operators.
Perldoc is excellent! use "perldoc perldoc" to learn how to use it.