If the number of entries for each domain is the same then the code below will interleave them for you

use warnings;
use strict;

my %domains;

while (my $line = <DATA>)
  {
  chomp $line;
  my ($name, $domain) = split /@/, $line;
  
  $domains{$domain} = [] if ! defined $domains{$domain};
  push @{$domains{$domain}}, $line;
  }

while (%domains)
  {
  for my $domain (keys %domains)
    {
    print ((shift @{$domains{$domain}}) . "\n");
    delete $domains{$domain} if ! @{$domains{$domain}};
    }
  }
  
__DATA__
ads@domain.com
ads@domain.com
ads@domain1.com
ads@domain1.com
ads@domain2.com
ads@domain2.com
[download]

Prints:

ads@domain.com
ads@domain1.com
ads@domain2.com
ads@domain.com
ads@domain1.com
ads@domain2.com
[download]