use POSIX 'ldexp';

    sub unpack_f_ieee754 {

        my @buf = unpack "C4", $_[0];
        my $mant;
        my $num;

        $mant  =  $buf[0];
        $mant |=  $buf[1]         << 8;
        $mant |= ($buf[2] & 0x7f) << 16;


        my $exp  = (($buf[3] & 0x7f) << 1) | (($buf[2] >> 7) & 0x01);
        my $sign = $buf[3] & 0x80;


        if ($exp == 0) {
            $num = ldexp($mant, -126 -23);
        }
        else {
            # restore implicit leading
            $mant |= 1 << 23;
            $num   = ldexp($mant, $exp -127 -23);
        }

        $num = -$num if $sign;

        return $num;
    }