A hash is something like an array. Say you have
%foo = (
'bar' => 'baz',
'qux' => 'quux',
);
Then do
register(%foo), and now the
@_ variable inside register might be like this:
$_[0] = 'bar';
$_[1] = 'baz';
$_[2] = 'qux';
$_[3] = 'quux';
It's almost like you called it with
register('bar' => 'baz', 'qux' => 'quux').
And remember that => is just a comma operator
which quotes the left argument, so that's like doing:
register('bar', 'baz', 'qux', 'quux').
Note how my assignment to %foo above is like assigning
a list ('bar' => 'baz', 'qux' => 'quux') to %foo.
So, what happened when you did
my (%error, %FORM) = @_;
is the elements of array @_ were assigned to %error,
where %error gobbles up all the list passed to it
because it doesn't know where to stop.