If I understand you correctly,

perl -pe"BEGIN{@ARGV = map{ glob} @ARGV; print'subscribername,telephon
+e,email,language', $/;}" *.txt > subscribers.all
[download]

would probably do the job.

But that said, if you simply want to concatenate all the files into one big file

copy con subscribers.all
subscribername,telephone,email,language
^Z
type *.txt >>subscribers.all
[download]

would also do it.