TIMTOWTDI: =)

  DB<270> @a{a..c}=1..3
 => (1, 2, 3)

  DB<271> @b{b..d}=6..8
 => (6, 7, 8)

  DB<272> @c{keys %a, keys %b}=()               ;# not needed 
 => (undef, undef, undef, undef, undef, undef)

  DB<273> $c{$_} += $a{$_} for keys %a
 => ""

  DB<274> $c{$_} += $b{$_} for keys %b
 => ""

  DB<275> \%c
 => { a => 1, b => 8, c => 10, d => 8 }
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line 272 is actually not needed and code doesn't cause warnings! :)