use strict;
use warnings;

my $counter = 0;

# get last step completed, if any
my $file = glob("S*.txt");
if (defined $file) {
  ($counter) = $file =~ m/S(\d+)\.txt/;
}

printf "Ready to start step %d\n",++$counter;
if ($counter == 1) {
  print "Just getting started.\n"
} elsif ($counter > 1 && $counter <= 4) {
  print "All kinds of interesting intermediate processing goes on here.\n"
} elsif ($counter == 5) {
  print "Finishing up.\n"
}

print "Completed step $counter.\n";
stepfile($counter);
# forget previous step, if any, now that this one is done
unlink $file if (defined $file);

sub stepfile {
  my $step = shift;
  my $filename = sprintf "S%06d.txt",$step;
  open my $tfh,'>',$filename;
}