Try replacing the following line:

if ($totalmass != 0){@wtPercent = map{$_ / $totalmass}@wtPureAmt}else{
+$statustext = "You need to give me numbers!"};
[download]

with its foreach equivalent:

if ($totalmass != 0){ 
    foreach (0 .. $#wtPercent) {
        $wtPercent[$_] = ($wtPureAmt[$_] / $totalmass); 
    }
} else {
    $statustext = "You need to give me numbers!"
};
[download]

I have not had time to fully read the docs but it seems as if reassigning the @wtPercent array using map is breaking its connection with -textvariable. I'm puzzled as to why this would be the case and would be interested to hear the full explanation from a more learned monk, but surprisingly the above works.
Very nice GUI by the way.