May be You should start reading WebAdverts installation file....

I try to make myself clearer.

Let say you want ads to display at the top of your page 1. You open your file "ads.pl" and you save it as "ads_top1.pl" in this file "ads_top1.pl" at: $advertzone=""; you enter "top1" so you have" $advertzone="top1"; for this specific file.

Now if you named your zone "a" you have to duplicate you file "ads.pl", rename the duplicated file "ads_a.pl" enter "a" in $advertzone="a"; and upload "ads_a.pl" on your server and call "ads_a.pl" with exec where you want on your pages.

In the specific file "ads.pl" under $advertzone=""; you enter nothing, you keep it like it is.

top1 is your zone for ads that will show up on top of your page 1. In webadvert under "Zone(s)" you enter "top1" (or "a") and banners will show up where you call ads_top1.pl with the exec function on your page.
On your server you will have to upload "ads.pl" AND "ads_top1.pl" AND all other duplicates of ads.pl that you will create (e.g.: ads_bottom1.pl, ads_top2.pl etc) in your folder webadvert.