I concur. It seems perfectly reasonable for the warning to appear when passing undef (literal or via variable) - '' ought to be used when you want the replacement to be an empty string. For the record, the documentation states:
An alternative to using substr() as an lvalue is to specify the replacement string as the 4th argument.
Nothing in there about undef being recognized as a warning-free shortcut for an empty string.

In reply to Re^2: Use of uninitialized value in substr by amedico
in thread Use of uninitialized value in substr by BrowserUk

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