Two things jump out at me immediately: First, you may want to turn off buffering.

open( OUT, ">blah.csv" ) or die "cant open out $!";
select OUT; $|++;
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Second, you should close your first filehandle (OUT) to blah.csv before opening a new handle (FH) to it. This is really just good practice given the way you are using the file.

Another option would be to reset the file pointer to the beginning of the file and just keep using the same filehandle.