sub buk {
	use List::Util qw[sum];
	my( $hashref, $start, $end ) = @_;

      # Added sanity check. Slowed it a tad
	return undef unless exists $hashref->{$start}
	                and exists $hashref->{$end};

	my @keys = grep{ $_ >= $start && $_ <= $end } keys %{ $hashref };

	return sum( @{ $hashref }{ @keys } ) / @keys;
}
__END__
P:\test>278169
0.503353665487943
0.503353665487943
       Rate orig  buk
orig 1.28/s   -- -42%
buk  2.23/s  73%   --

##</code><code>##

sub buk2 {
	use List::Util qw[sum];
	return undef unless exists $_[0]->{ $_[1] }
	                and exists $_[0]->{ $_[2] };

	return sum( @{ $_[0] }{ $_[1] .. $_[2] } ) / ( $_[2] - $_[1] + 1 );
}

__END__
P:\test>278169
0.503353665487943
0.503353665487943
0.503353665487943
       Rate orig  buk buk2
orig 1.30/s   -- -41% -81%
buk  2.19/s  69%   -- -67%
buk2 6.70/s 417% 206%   --