XQuery syntax error

You don't need the curly braces if you aren't embedding it in your own node:

with mytable (xml_file) as (
  select xmltype('<pod><trader cdisp="AB1111">
<idimpp num="1234">
<hdr>
<odn>567</odn>
<pod>XX999</pod>
</hdr>
</idimpp>
</trader>
</pod>')
  from dual
)
SELECT XMLQUERY
  ('for $e in //trader
    where $e//pod="XX999"
    order by $e
    return $e'
  passing MY_T.XML_FILE
  returning content
)
FROM MYTABLE MY_T;

XMLQUERY('FOR$EIN//TRADERWHERE$E//POD="XX999"ORDERBY$ERETURN$E'PASSINGMY_T.XML_FILERETURNINGCONTENT)
--------------------------------------------------------------------------------
<trader cdisp="AB1111"><idimpp num="1234"><hdr><odn>567</odn><pod>XX999</pod></hdr></idimpp></trader>

From the documentation on XQuery expressions:

Computed (dynamic) constructions – You can construct XML data at run time using computed values. For example, the following XQuery expression constructs this XML data: <foo toto="5"><bar>tata titi</bar> why? </foo>.

<foo>attribute toto {2+3},
     element bar {"tata", "titi"},
     text {" why? "}</foo>

In this example, element foo is a direct construction; the other constructions are computed. In practice, the arguments to computed constructors are not literals (such as toto and "tata"), but expressions to be evaluated (such as 2+3). Both the name and the value arguments of an element or attribute constructor can be computed. Braces ({, }) are used to mark off an XQuery expression to be evaluated.

In your first query pod is a direct construction and ${e} is computed, so the braces are necessary. In the second version you're referring directly to $e outside any construction, so there is no evaluation, and therefore no braces are required (or allowed).