Don't use $y-=1900, use $y-1900 instead.  As you have it, your value will be decremented twice, yielding an invalid year the second time.  (I'm supposing you're setting $y once somewhere at the beginning — though you haven't shown it...)

P.S.: In case anyone wonders (as I did) why when the module croaks the year value is reported as 109 (though it isn't)...  that's because the module does:

    ...
    $year += 1900;
    croak "Cannot handle date ($sec, $min, $hour, $mday, $month, $year
+)";
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