The specifications for XML 1.0 are set forth by the World Wide Web Consortium here. These specs state:

Enumerated Attribute Types
...
[59]    Enumeration    ::=    '(' S? Nmtoken (S? '|' S? Nmtoken)* S? '
+)' [VC: Enumeration] 
...
Examples of attribute-list declarations:
...
<!ATTLIST list
          type    (bullets|ordered|glossary)  "ordered">
...
[download]

My reading of the XML specifications is that pipes in attribute-list declarations should not have spaces surrounding them.

Similarly, in XML 1.0 one ought not to surround pipes with spaces in element type declarations either:

Examples of element type declarations:
...
<!ELEMENT p (#PCDATA|emph)* >
[download]

It is unfortunate that attr test contains what may be a malformed XML Document Type Declaration. However, the XML specifications contain no requirements for user-friendly error handling. All an XML parser has to do when encountering bad XML is stop. It need do no more.

Update: Our resident XML guru Mirod has pointed out to me that the S? in the XML rule I quoted means that the pipe can, indeed be surrounded by whitespace. He opines that there is an error, but that it is likely in XML::Parser on which XML::DOM is built.

Mirod points out that according to Rule 3

(see rule 3: S ::= (#x20 | #x9 | #xD | #xA)+ >

Hence, S? means there can be an optional space around the '|'

Moral. I should learn to read XML specs better. Thanks to mirod for correcting this error.