my $rpt_tmpl = "cnc1_rpt" . $rpt_id . "_summary.tmpl";

# another way to achieve the same result:
my $rpt_tmpl = "cnc1_rpt@{[$rpt_id]}_summary.tmpl";

# and yet anther way
my $rpt_tmpl = sprintf("cnc1_rpt%d_summary.tmpl", $rpt_id);