Short version (it's all I have time for): don't. Use regexen. When you're using perl, shelling out is an expensive and unnecessary option in most cases.
It looks as tho you should read about regexen -- and specifically, substitution -- in a tutorial right here; that is, in the Tutorials section.
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Want more info? How to link or
or How to display code and escape characters
are good places to start.