for ($str) {
     s/0(0*)$/1$1/
     or s/1(0*)1$/01$1/
     or return;
   }


##</code><code>##

sub nextmask {
     if ($_[0] =~ s/0(0*)$/1$1/) {1;}
     elsif ($_[0] =~ s/1(0*)1$/01$1/) {2;}
     else {0;}
}

my @factors = qw(2 3 5 7);
my $mask = '0' x scalar(@factors);
while (my $transtype = nextmask($mask))
{

  print "BREAK\n" if ($transtype > 1);
  # do stuff 
  print join " ", 
        map { substr($mask, $_, 1) ? $factors[$_] : ' ' }
                     0 .. $#factors;
  print "\n";
}


##</code><code>##

--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/