so you bneed to ask "why is Date::Calc returning a nonense value?"

No. The question to ask is What is Date::Calc returning?. What you do know is that it appears as -0 when printed. It could be a string. It could be an overloaded object. But since Date::Calc deals with numbers, it's probably a number. Now, we all know that floats can't always be represented exactly. So, could it be that -0 represents a small negative value, small enough that it stringifies to -0? Let's try:

    $ perl -wle 'print -1 / (10 ** 5000)'
    -0
    $
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I guess it can.

Note that the code of the OP was using -0 literals, which are a totally different kind of beast.

Abigail