You mistake is (almost assuredly) in assigning it to a hash. Rather than code that says:

my %param = URI->new('/?foo=Zoo&foo=aha')->query_form;
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you should respect that you have multiple values associated with a given key, and use hashes of arrays (perllol, HASHES OF ARRAYS):

my @list = URI->new('/?foo=Zoo&foo=aha')->query_form;
my %param;
while (@list) {
    push @{$param{shift @list}}, shift @list;
}
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or something similar.