The problem is that param() is sensitive to context. If you call it in a list context (which you are), and there's no value, then it returns an empty list (that is, zero items). This is different than returning undef (which would be one item). If you really want just one item from param(), call it in scalar context:

$template->param( STYLE_SHEET       => "$STYLE_SHEET",
                  SCRIPT_LOCATION   => "$SCRIPT_LOCATION",
                  MEMBER_NAME       => scalar( $q->param('realname') )
                 );
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