Brute-force algorithm:

sub is_power {
    # Some algorithm to determine if a number is a power of two.
}

sub closest_power_of_two_brute {
    my $m = my $n = shift;

    return $n if is_power( $n, 2 );

    while (1) {
        return $n if is_power( $++n, 2 );
        return $m if is_power( $--m, 2 );
    }
}