Just change ">>= 1" to "/= 2" plus similar changes to get:

$tmp = @ARGV[0];
$i = 0;

while( 1 <= $tmp )
{
    $tmp /= 2;
    $i += 1;
}
print 2 ** $i;
[download]

and you are good up to about 52 bits (instead of 32), probably quite a bit farther (depends exactly how the round-off enters into the calculations). I didn't get even a one-bit error until I tried that on 36028797018963967 (which returned twice of one more than that number, instead of just one more than that number).

I'd do this more like:

sub nextpow2 {
    my $s= shift(@_);
    my $p= 1;
    $p *= 2   while  $p <= $s;
    return $p;
}
[download]

which exhibits the same problem but only because you can't represent 36028797018963967 exactly as a floating point number.

So this works for quite high powers of two provided your input isn't just under a large power of two.

        - tye (don't fear the floating point)