the idea of wantarray is a little bit different.
e.g. a subroutine is using it to determ if the caller wants an array or a scalar

sub foo {
   return wantarray ? @_ : "@_";
}

my @bar = foo(1, 2, 3); # @bar = (1, 2, 3);
my $bar = foo(1, 2, 3); # $bar = "1 2 3"
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but

my @tmp = $this->{'ApacheRequest'}->param( "checkbox_field");
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should do what you want