Well, as a workaround i'd say something like this should work:

my tmp = new File::Temp( TEMPLATE => "temp".$$."XXXXX",
                                   DIR => '/tmp',
                                   SUFFIX => '.dat');
[download]

At least you won't have to modify File::Temp.

Though I would expect File::Temp to do something similar for me, I'm not sure that it actually guarantees a unique filename for forked() siblings. Maybe you should file a bug report. (don't forget a working example demonstrating the bug)