my $rep_date = $report{$host}{$pol}{$sched}{$date};

if ( defined ${ $rep_date } )
{
  ${ $rep_date } .= $error;
}
else
{
  ${ $rep_date } = $error;
}
#
#  ... or ...
#${ $rep_date } =
#  defined ${ $rep_date } ? ${ $rep_date } . $error
#  : $error
#  ;