writting anonymous variables in a file

Hello,

I want to write atoms with anonymous variables,
like p(_, _), in a file,
but in the file, each anonymous variable (_) have been replaced
by a named anonymous variable, for example _906541.

Unfornatly, with big data, the same named anonymous variable
appears more than once in the file.

How can I do write strictly anonymous variables ?

Thanks a lot

Dominique