From the "perlform" documentation:

NOTES
       Because the values line may contain arbitrary expressions (for at fields, not caret fields), you can farm out more sophisticated processing
       to other functions, like sprintf() or one of your own.  For example:

           format Ident =
               @<<<<<<<<<<<<<<<
               &commify($n)
           .

and for a discussion on the implementation of "commify", see this thread.