The problem you have with tabs is that they aren't exactly 4 (or 8) spaces. They are upto the next TABSTOP...

   |   |   |   |   |   |   |   |   |   |
1234567890123456789012345678901234567890
this is\ta test
this is    a test
this was\ta test
this was   a test
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As you can see from the above, the tab character represents a different amount of spaces depending on how close to the next tabstop you are.

Simple substitution of \t for \s{4} will not work. You must know how many spaces are necessary to reach the next stop.

Text::Tabs will correctly expand and contract tabs for you.