Refining what InfiniteSilence said and applying to the OP examples, I'd go with:

sub part0 { ... }
sub part1 { ... }
if ( $condition1 ) {
  {
    part0();
    last if $condition2;
    part1();
  }
  --$loop;
}
[download]

Then all of the condition and decrement logic is, visually, close together, so easier to understand.