Once I fixed the error revealed by using

use strict;
use warnings;
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mydata_$GPVTG.txt successfully gets created from mydata.txt.

You have other problems, though. Starting with foreach my $key(%files) iterating over keys and values. I'll give you a chance to try to fix them.

By the way,

local *FH; 
open (FH, ...) or die;
$files{$ffc} = *FH;
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can be written as

open ($files{$ffc}, ...) or die;
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