That should be q> (big-endian byte order), not q (native byte order).

Furthermore, it would be better to pad out the hex number in case fewer than 16 digits are provided. 

sub hex_to_signed_quad {
    return unpack 'q>', substr(('0'x16).$_[0], -16);
}

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In reply to Re^2: -1 from hex ffffffffffffffff ? by ikegami
in thread -1 from hex ffffffffffffffff ? by Spooky

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