Thanks! I knew I was missing the obvious here. A modulus did the trick:

    for my $i (1 .. $itemnum){
        push (@tmp_row_items, $q->td($q->textfield( -name => "$champar
+ray[$productcount][0]$i", -default => "$champarray[$productcount][$i]
+", -size => 25)));
        if ($i % 5 == 0) {    
            push (@rows, $q->td(@tmp_row_items));
            @tmp_row_items = ();
        }
    }
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