If your lucky enough to have access to the XS version of List::Util this would probably run much faster still.

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%   --
[download]

Second attempt (417%)

Once I noticed that your code assumes that a contiguous range of keys will exist between the start and end values, it can go much quicker.

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%   --
[download]