map and a little casting magic does the trick. Note the %{$a[$_]} that turns the hash ref returned by $a[$_] into a hash.

BTW, you should perhaps avoid using a and b - even when they are not scalars they can cause confusion due to $a and $b being used for sort parameters. (Yes, I know this is a trivial sample, but good habits are good habits.)

use strict;
use warnings;
use Data::Dump::Streamer;

my @a_ = ({ domain => 'adomain.com',
          data => 'adata',
        },
        { domain => 'bdomain.com',
          data => 'bdata',
        });

my @b_ = ({ domain => 'adomain.com',
          otherdata => 'astuff',
        },
        { domain => 'bdomain.com',
          otherdata => 'bstuff',
        });

my @c = map {{%{$a_[$_]}, %{$b_[$_]}}} 0..$#a_;

Dump (\@c);
[download]

Prints:

$ARRAY1 = [
            {
              data      => 'adata',
              domain    => 'adomain.com',
              otherdata => 'astuff'
            },
            {
              data      => 'bdata',
              domain    => 'bdomain.com',
              otherdata => 'bstuff'
            }
          ];
[download]