I don't know much about Tcl, but I thought everything was a string, not a float? It appears to work with integers just fine:

  $ echo 'puts [expr 2 ** 128 + 1]' | tclsh
  340282366920938463463374607431768211457

  $ python3 -c 'print(2 ** 128 + 1)'
  340282366920938463463374607431768211457

  $ node -p 'Math.pow(2, 128) + 1'
  3.402823669209385e+38