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

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