Using your code above as is with the exceptions of adding a shebang line and using $ARGV[0] for the filename and it crunches with

P:\test>523624-a 1000000.dat
syswrite() on closed filehandle $fh at P:\test\523624-a.pl line 26.
Bad file descriptor at P:\test\523624-a.pl line 26.
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Which I do not understand at all. Any thoughts?

Update: Ok. I couldn't see it for looking, but you have a comma instead of a semicolon on the syswrite line.