that's what you are looking for?

  DB<253> @a{a..c}=1..3
 => (1, 2, 3)

  DB<254> @b{b..d}=6..8
 => (6, 7, 8)

  DB<255> $c{$_}=$a{$_}+$b{$_} for (keys %a,keys %b)
 => ""

  DB<256> \%c
 => { a => 1, b => 8, c => 10, d => 8 }
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NB: you might need to set no warnings 'uninitialized' in a scope surrounding line 255.

update

I just noticed that, while producing correct results, this code will be a bit redundant, cause it repeats operations (here for b and c)

If this is really an issue, do an exists $c{$_} check before adding or apply uniq from List::MoreUtils.