If you look back at the warning message you got, you see that it mentions "_size" as the string that is not numeric. Your code snippet will only set $1 to a string of digits, not to the string "_size", right? So, I don't think your code snippet is the problem.

Regarding the line above: somehow, $self->{_size} is getting set to the string "_size", instead of to a number.

Can you post more of the code from Spreadsheet/Format.pm? What method is line 225 part of? We'll just have to keep poking around the code until we figure this out.