Hello!

I'm assuming that you call 'element' a value of $hash{$key} (because in a canonical way of speaking 'element' is a pair of key and value in a hash).

1. Detect the element is an ARRAY... in perl

foreach my $element (values %hash) {
  if (ref $element eq ref []) {
  }
  # OR 
  if (ref $element eq 'ARRAY') {
  }
}
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2. Acknowledge the number of elements in each hash

my $number_of_elements = scalar keys %hash;
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3. Avoid having the two ARRAY references within a hash element

There can be only one reference in a hash value. Did you mean "avoid pushing ARRAY into hash element, if it's already an ARRAY ref"?

my %hash;
foreach my $pair (qw/1_one 1_two 2_one 2_two/) {
  my ($key, $val) = split('_', $pair);
  if (not exists $hash{$key}) {
    $hash{$key} = [];
  }
  push @{ $hash{$key} }, $val;
}
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