Creating diagonal array but retaining first n columns.

This is sort of a continuation of  question I previously asked, which was:

https://www.experts-exchange.com/questions/21857321/combine-2D-arrays-in-a-funky-way.html

Please see the comments after the accepted answer posting.

An Expert provided this code:

for( \@ary1, \@ary2, \@ary3 ){
    my $right=@{$_->[0]};
    my $left=@combinedary&&@{$combinedary[0]};
    push @$_,(0)x$right for @combinedary;
    push @combinedary,[(0)x$left, @$_] for @$_;
}

The idea is to combined multiple arrays and create a "diagonal" of values and fill with zeros everywhere else, which is exactly what the above code accomplishes.  But for this question, I would like to retain the first n columns.  Please see below for a detailed description.  This example only combines two arrays but the solution should apply to two or more arrays.

@ary1 = (
[1,2,3,4]
);

@ary2 = (
[10,9,8]
);

If n==2, then the final array would be:
@final = (
[1,2,3,4,0],
[10,9,0,0,8]
),

if n==1, then the final array would be:
@final = (
[1,2,3,4,0,0],
[10,0,0,0,9,8]
);

Another way to think of this would be to add @ary1 and @ary2:

@combined = (
[1,2,3,4],
[10,9,8]
);

Then split after n columns:
n==2:
@left = (
[1,2],
[10,9]
);

@right = (
[3,4],
[8]
);

Apply the above zero padding method to @right but leave @left alone:
@right_result = (
[3,4,0],
[0,0,8]
);

Finally, put @left and @right_result together:
@rightleft = (
[1,2,3,4,0]
[10,9,0,0,8]
);