my @customers= $dbh->selectall_arrayref(<<SQL, { Slice => {} });
 select deb.id
   from fte_intra.debiteuren deb
  where deb.type='sms'
    AND (deb.Afloop_contract='0000-00-00' OR month(deb.Afloop_contract)='$lastmonth')
  group by deb.id
  order by deb.id
SQL

for my $ref (@customers) {
    {
    $invoice_cid = $ref->{'id'};
    my $statement = "select mc.cid,mc.mail_month from gmsms.mail_count mc where mc.cid = '$invoice_cid' AND mc.month='$lastmonth' AND mc.year='$yearG'";
    my $sth = $DBH->prepare($statement);
    ...
};

##</code><code>##

select
    deb.id
  , mc.cid
  , mc.mail_month
 from fte_intra.debiteuren deb
left join gmsms.mail_count mc on (mc.cid = deb.id)
where
      deb.type='sms'
  AND (deb.Afloop_contract='0000-00-00' OR month(deb.Afloop_contract)='$lastmonth')
  AND mc.month='$lastmonth'
  AND mc.year='$yearG'
group by deb.id
order by deb.id