I like the procedural form of grid, so I don't have to care about -row and -column. Here's your sample rewritten:

foreach (sort keys(%density)) {
    Tk::grid(
        $mb->Label(
            -text => $_,
            -anchor => "w",
        ),
        $mb->Entry(
            -textvariable => \$density{$_},
            -width => 5
        ),
        -sticky => "ew",
       );
}
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