Is it returning an array or an array reference as your last code snippet seems to indicate? You can use ref to determine whether a scalar is a reference and if so what kind.

my $ref = $this->{'ApacheRequest'}->param( "checkbox_field");
my @tmp = ref($ref)?@$ref:$ref;
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wantarray is actually intended to be used in the called subroutine (param in this case) so it can tell whether you're calling it in scalar context or array context.