Is this a Perl question? In order to get a useful answer you probably have to provide some details about your app.

If your question is simply: on a screen of width $w and height $h and a centered window of 80x25, how wide is my left and top margin (or bottom?), the following should be helpful:

$left = ($w - 80) / 2;
$top = ($h - 25) / 2;
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which both might be negative on a small screen.