It sounds like you don't need a regexp at all, just 
substr($_, 10, 1)=',';

[download]
(watch out for 0-based offsets!) If you really want a regexp solution, you could do something like 
s/^(.{10})./$1,/;

[download]

In reply to Re: Positional regex statements by Eimi Metamorphoumai
in thread Positional regex statements by jcoxen

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