#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11121888
use warnings;

print "$_\n" for strings( 4, 10 );

sub strings
  {
  my ($ones, $length) = @_;
  $length <= $ones ? 1 x $length : $ones < 1 ? 0 x $length :
    ( map("1$_", strings( $ones - 1, $length - 1 ) ),
      map "0$_", strings( $ones, $length - 1 ) )
  }