sub combinatoric($$); # prototype forward reference

sub combinatoric( $$ )
{
   my ( $zero, $one ) = @_;
 
   my @ret = ();
   if ( $one == 0 )
   {
      # no more ones left to select, the rest is zeroes.
      push @ret, scalar ( '0' x $zero );
   }
   elsif ( $zero == 0 )
   {
      # no more zeroes left to select, the rest is ones.
      push @ret, scalar ( '1' x $one );
   }
   else
   {
      # take a zero from the pile, and compute what's left
      push @ret, "0$_" foreach combinatoric( $zero-1, $one );
      # take a one from the pile, and compute what's left
      push @ret, "1$_" foreach combinatoric( $zero, $one-1 );
   }
   return @ret;
}

print "$_\n" foreach combinatoric( 10, 10 );