Have you considered what your code will do when faced with a query string like:

key1=val1&key2=val2a&key2=val2b
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which is a perfectly valid query string. Your %params hash will end up looking like:

%param = (key1 => 'val1', key2 => 'val2b');
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The first value of key2 will have been lost. CGI.pm's parameter routines handle this correctly. If you have a key which has mulitple values, then the param function will return a list of values.