For those not familiar with graphical interfaces, this is often called a "callback". In any case, one must assume that your long running process has a way of knowing how far along it is, and you'll have to have various points in the process explicitly call the callback with the current status report.

sub statusPrint {
# code to update status
  print shift;
}

sub longprocess {
   my &status = shift;
   #do stuff
   &status("10%");

   #more stuff

   &status("20%");
}

call it with:
longprocess (\&statusPrint);
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