Besides what chipmunk said (about ten seconds before I did), I got a couple of non-numeric errors about the concatenation where you add one to $x. here's my rewrite of nested():

sub nested {
    my $str = $q->start_table( -border => 1);
    for (my $x = 4; $x < 8; $x++) {
        $str .= $q->Tr(
            $q->td("cell ". $x),
            $q->td("cell ". $x++)
        );
    }
    return $str . $q->end_table;
}
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