#!/usr/bin/perl

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

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

sub strings
  {
  my @strings = '';
  my $zeros = $_[0] - $_[1];
  my $ones = $_[1];
  @strings = map +(
    ($@ = tr/1//) < $ones ? $_.1 : (),
    length($_) - $@ < $zeros ? $_.0 : ()
    ), @strings for 1 .. $_[0];
  return @strings;
  }