Perhaps a diagram will help.

print "main start\n";
&mySub(4);
print "main end\n";

sub mySub
{
     my ($oldValue, $callDepth, @leftOvers) = @_;
     if (!defined $oldValue) { $oldValue = 0; }
     if (!defined $callDepth) { $callDepth = 0; }

     # Increment call depth each time we get into mySub()
     $callDepth++;
     my $indentSize = $callDepth * 2;
     my $indentSpaces = sprintf("%${indentSize}s", "");

     # Show we've arrived -- indent according to call depth
     print "${indentSpaces}mySub($oldValue) BEGIN\n";

     # Cut value in half and recurse until zero
     my $newValue = int($oldValue / 2);
     if ($newValue > 0)
     {
          &mySub($newValue, $callDepth);
     }

     # Show we've returned -- indent according to call depth
     print "${indentSpaces}mySub($oldValue) END\n";

     # Bye-bye
     return;
}
[download]

Results:

main start
  mySub(4) BEGIN
    mySub(2) BEGIN
      mySub(1) BEGIN
      mySub(1) END
    mySub(2) END
  mySub(4) END
main end
[download]

Diagram of how it works (conceptually, if not implementorily):

main start	main() mySub()     ^     |     +-- You are here         You call mySub(4)
mySub(4) BEGIN	main() mySub(4)             ^             |             +-- You are here                 You call mySub(2)
mySub(2) BEGIN	main() mySub(4)             mySub(2)               ^               |               +-- You are here                   You call mySub(1)
mySub(1) BEGIN             mySub(1) END	main() mySub(4)             mySub(2)               mySub(1)                 ^                 |                 +-- You are here
mySub(2) END	main() mySub(4)             mySub(2)               ^               |               +-- You are here
mySub(4) END	main() mySub(4)             ^             |             +-- You are here
main end	main() mySub()     ^     |     +-- You are here