The problem is that you reopen c.txt for output '>c.txt', which overwrites the file, and then print to the terminal.

If you wish to keep the original contents of c.txt and append to it, reopen the file for append '>>c.txt'.

And either way, you need to say where you want your output to go by adding the file handle to the print line

foreach (keys %tagdict){
    print CF values %tagdict if exists $tagcorpus{$_};
} #       ^^ Here!
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