What should decent code look like??

I looked at your first line:

if ($#ARGV != 2){die "usage $0 filenamein filenameout\n";}
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You require the program to have exactly three arguments. Your die message suggest two are needed. Looking through the code, it turns out you indeed use all three. But the last one almost at the bottom. Better to assign your arguments to meaningful variables at the top of your program.

As for the rest of the code, the almost total lack of indentation makes it too hard to read. I'm not going to bother.