sub IEEE80toIEEE64 {
    my( $discard, $mantissa, $hidden, $exponent, $sign ) =
        unpack 'a11 a52 a1 a15 a1', unpack 'b80', $_[ 0 ];
    $exponent = unpack( 'v', pack 'b15', $exponent ) - 16383 + 1023;
    $exponent = 32767, $mantissa = '0' x 52
        if $exponent < 0 or $exponent > 2047;
    $exponent = unpack 'b11', pack 'v', $exponent;
    return unpack 'd', pack 'b64', $mantissa . $exponent . $sign;
}