figured it out... you can do number to number or cell reference to cell reference substitutions so that the following will work:

my $formula1 = $ws->store_formula('=(12*9999)+MONTH($L39-1)-MONTH(A92)
++1');
 
  my $diff=111;
  $ws->repeat_formula($row, $col, $formula1, $fmt->{'number'},
                                     qw/^9999$/, $diff,
                                     qw/^A92$/, 'A' . $current_row
                        );
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