It's the fact that you can name both the key and the value from the hash as you iterate so you do not have to use $hash{$key}, but instead have just $value. Which is especially handy if that hash is not in a variable, but rather is part of a more complex datstructure. So that instead of

foreach my $key (keys %{$data->{blah}[$idx]{blih}}) {
  print "$key : $data->{blah}[$idx]{blih}{$key}\n";
}
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while (my ($key, $value) = each %{$data->{blah}[$idx]{blih}}) {
  print "$key : $value\n";
}
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In that example it did not make a big difference especially because the value was used only once, but it may be handy. Especially if you find yourself doing something like

foreach my $key (keys %{$data->{blah}[$idx]{blih}}) {
  my $value = $data->{blah}[$idx]{blih}{$key};
  ...
}
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