use List::Util qw(sum);
sub average {
    my ($data, $first, $last) = @_;
    my @goodkeys = grep exists $data->{$_}, $first .. $last;
    sum( @{$data}{@goodkeys} ) / @goodkeys;
}

##</code><code>##

use List::Util qw(sum);
sub average {
    my ($data, $first, $last) = @_;
    my @values = grep defined, @{$data}{$first .. $last};
    sum(@values) / @values;
}