Or you could use a slice instead of the inner loop. The following piece is a rewrite, that is easily extended to more than merging the result of two subs:

my $part1 = Sub1();
my $part2 = Sub2();

my $ans = {}; # where the final answer will live

foreach my $part ($part1, $part2) { # loop over parts
  foreach my $k (keys %$part) {
     my $inner = $part->{$k};
     @{$ans->{$k}}{keys %$inner} = values %$inner;
  }
}

use Data::Dump qw(dump);
print dump $ans; # that's what you got
[download]

Obviously, this merge only works at two-level hashes.