(Comming very late to the discussion via "Random Node" ...) If you do find this in some code and do not have time to switch over to $q->param("Foo") or something or if you feel too strongly that you need to save a few keystrokes, you may do this:

${$_} = CGI::param($_) for qw(Username Password Email);
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This way you know what parameters you expect and what variables will get their values from the query. No surprises with variables being overwritten by an unexpected query parameter.