use Time::HiRes;
my $ENABLE_PROFILING = 1;
my $exit_genchars_time = 0;

...

sub exit_genchars {

 my $temp_time = Time::HiRes::time() 
   if $ENABLE_PROFILING;
 log_event("Exit called\n");
 $exit_genchars_time += 
   Time::HiRes::time() - $temp_time if $ENABLE_PROFILING;
 exit(0);
}

...

sub END {
  if ($ENABLE_PROFILING) {
    print "Spent " . sprintf("%.2f") . 
     " seconds inside exit_genchars\n";
    ...
    ...
  }   
}