Assuming that you've already used CGI.pm and declared the hash. And also that the text field is called hashdata...

foreach (split /,\s*/, param('hashdata')) {
  my ($k, $v) = split(/=>/);
  $hash{$k} = $v;
}
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If you know the hash is going to be empty at the start, then it gets a bit simpler...

%hash = map { split /=>/ } 
        split /,\s*/, param('hashdata');
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