hardburn is right about what constitutes a whitespace, but I suspect that star7 may not be using the term correctly.
I'm also confused, star7, when you say tabs are the problem. You seem to be putting tabs into your document. If they are the problem, wouldn't you be trying to remove them?

$str =~ s/\t/ /g;

Replacing each tab with a space. (not whitespace)

-ted-