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The inverse of C<binstr754_from_double()>: it takes a string representing the 64 bits
of the IEEE754 double value, and converts it back to a perl floating-point value.

    print binstr754_to_double('0100000000101001110000000000000000000000000000000000000000000000');
    12.875

=cut
# Perl 5.10 introduced the ">" and "<" modifiers for pack which can be used to
# force a specific endianness.
if( $] lt '5.010' ) {
    my $str = join('', unpack("H*", pack 'L' => 0x12345678));
    if('78563412' eq $str) {        # little endian, so reverse byteorder
        *hexstr754_from_double  = sub { return uc unpack('H*' => reverse pack 'd'  => shift); };
        *binstr754_from_double  = sub { return uc unpack('B*' => reverse pack 'd'  => shift); };

        *hexstr754_to_double    = sub { return    unpack('d'  => reverse pack 'H*' => shift); };
        *binstr754_to_double    = sub { return    unpack('d'  => reverse pack 'B*' => shift); };

        *arr2x32b_from_double   = sub { return    unpack('N2' => reverse pack 'd'  => shift); };
    } elsif('12345678' eq $str) {   # big endian, so keep default byteorder