c:\@Work\Perl\monks>perl -wMstrict -MO=Deparse,-p -le
"my %h1 = qw( a 1  b 2  c 3      );
 my %h2 = qw(      b 7  c 8  d 4 );
 my %h3 = map { $_ => $h1{$_} // 0 + $h2{$_} // 0 } keys %h1, keys %h2;
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
(my(%h1) = ('a', '1', 'b', '2', 'c', '3'));
(my(%h2) = ('b', '7', 'c', '8', 'd', '4'));
(my(%h3) = map({($_, (($h1{$_} // (0 + $h2{$_})) // 0));} keys(%h1), keys(%h2)));
-e syntax OK