phasers.pm

my $r = ''; 

sub foo {
  UNDO { $r ~= "U1"}
  KEEP { $r ~= "R1"}
  CATCH {
    if (1) {
      UNDO { $r ~= "U3"}
      KEEP { $r ~= "R3"}
      return True;
    }   
  }
  if (1) {
    UNDO { $r ~= "U2"}
    KEEP { $r ~= "R2"}
    die("foo");
  }
}

foo();
say $r;