my $current_month = 11;
my $num_days_of_supp = 120;

my @months_of_support = ( ($current_month-($num_days_of_supp/30)+1) .. $current_month );
my $sql = 'SELECT GREATEST(ROUND((SUM'
  . join( '+', map { sprintf "mth_%02i", $_ } ( 0 .. 12 ) )
  . ") / 365 ) * $num_days_of_supp ), ROUND (SUM ("
  . join('+', map { sprintf "mth_%02i", $_ } @months_of_support)
  . ')))  QTY, b. p_dlrn, b.p_dlrsplrcode '
  . <<EOF;
FROM ...
EOF