c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my %h1 = qw(foo 3  bar 99);
 my %h2 = qw(foo 5  bar 87);
 ;;
 die 'hashes unequal: merging invalid' if keys %h1 != keys %h2;
 ;;
 for my $k (keys %h1) {
   die qq{key '$k' not found: merging invalid} unless exists $h2{$k};
   $h1{$k} += $h2{$k}
   }
 ;;
 dd \%h1;
"
{ bar => 186, foo => 8 }
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Update: Hey, isn't this a FAQ somewhere? Can't find it ATM.