so I did a little testing

these examples

lanx@nc10-ubuntu:/tmp$ cat end.pl 
print <DATA>;
__END__
end
lanx@nc10-ubuntu:/tmp$ cat data.pl 
print <DATA>;
__DATA__
data
[download]

produce identical op-codes.

I'm not aware of a mechanisms to access the content after __DATA__/__END__ other than reading from the file-handle of the source (i.e. <DATA> ) at compile/run time.

Since B::Deparse is meant to analyze single files and both stop-markers are only different in a multi-file situation you might have entered an edge-case where B::Deparse has no possibility to see any difference.

I.o.W. can you produce an example where there is a real difference between __DATA__ and __END__ where B::Deparse actually fails?