You're using cmp which compares strings, you need to compare numbers so you should use <=>
This should do it for you

my %hash = map { 
    "$url?pg=$in{pg};load=$in{load};l=$in{l};max=$_$include_sess_id"
    , "$_" } 
    (1, 5, 10, 15, 20, 25, 35, 50, 75, 100, 200, 300, 400, 500);

$change_max .= popup_menu(-name=>"change_max",
              -class=>"formfield",
              -values=> [sort {$a <=> $b} keys  %hash],
              -labels=> \%hash,
              -class=>"formfield",
              -default=>$maxlimit_display,
              -onchange=>"window.location=(document.max.change_max.val
+ue)"
);
[download]

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