When a value becomes too big to store as a signed integer, perl automatically upgrades it to an unsigned integer (technically it sets the SVf_IVisUV flag on the SV). If the value becomes too large even for that, then the SV is upgraded to an NV. This is all automatic and you shouldn't have to do anything yourself (unless you're using an ancient version of perl).
Try running
perl -MDevel::Peek -e'$x=(1<<60)-1; for (1..6) { Dump $x; $x = $x *2 +
+ 1}'
If you want something more than this, you'll have to explain your need better.
Dave.