Allthough I originally wrote a module like
Test::Warn, I wasn't aware of that behaviour.
However
perldiag describes it as a kind of optimization:
Use of uninitialized value%s
(W uninitialized) An undefined value was used as if it were already
defined. It was interpreted as a "" or a 0, but maybe it was a
mistake. To suppress this warning assign a defined value to your
variables.
To help you figure out what was undefined, perl will try to tell
you the name of the variable (if any) that was undefined. In some
cases it cannot do this, so it also tells you what operation you
used the undefined value in. Note, however, that perl optimizes
your program and the operation displayed in the warning may not
necessarily appear literally in your program. For example, "that
$foo" is usually optimized into ""that " . $foo", and the warning
will refer to the "concatenation (.)" operator, even though there
is no "." in your program.