For the record your second solution corrupts the hash with keys whose value is undef.

Eh? Extraordinarily strange. So a slice lookup autovivifies elements, while a single-element lookup doesn't. Feels like a bug, if you ask me.. certainly counterintuitive. Easy enough to fix, though.

use List::Util qw(sum);
sub average {
    my ($data, $first, $last) = @_;
    my @values = map { local $_ = $data->{$_}; defined() ? $_ : () } $
+first .. $last;
    sum(@values) / @values;
}
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Makeshifts last the longest.