#!/usr/local/bin/perl

use strict;
use warnings;

use Data::Dumper;


sub find_numbers {

  my ($amount, $map)=@_;

  my $start = 11;
  my @found;

  while ($amount--) {
    while ($map->{$start}) {
      ++$start;
    }
    push @found, $start++;
  }

  @found;

}

my %map = (
          '11' => 1,
          '12' => 1,
          '15' => 1,);

my @free_numbers = find_numbers(4, \%map);

die Dumper(\@free_numbers, \%map);