[johngg@ovs276 perl]$ ls
abc.txt  def.txt
[johngg@ovs276 perl]$ head -99 *
==> abc.txt <==
test text line 1
test text line 2
test text line 3

==> def.txt <==
test text line 1
test text line 2
test text line 3
[johngg@ovs276 perl]$ perl -i.BAK -pe 'BEGIN { $\ = ",$ARGV[0]\n" } chomp' *.txt
[johngg@ovs276 perl]$ ls
abc.txt      abc.txt.BAK  def.txt      def.txt.BAK
[johngg@ovs276 perl]$ head -99 *
==> abc.txt <==
test text line 1,abc.txt
test text line 2,abc.txt
test text line 3,abc.txt

==> abc.txt.BAK <==
test text line 1
test text line 2
test text line 3

==> def.txt <==
test text line 1,abc.txt
test text line 2,abc.txt
test text line 3,abc.txt

==> def.txt.BAK <==
test text line 1
test text line 2
test text line 3
[johngg@ovs276 perl]$
[johngg@ovs276 perl]$
[johngg@ovs276 perl]$ mv abc.txt.BAK abc.txt
[johngg@ovs276 perl]$ mv def.txt.BAK def.txt
[johngg@ovs276 perl]$ ls
abc.txt  def.txt
[johngg@ovs276 perl]$ head -99 *
==> abc.txt <==
test text line 1
test text line 2
test text line 3

==> def.txt <==
test text line 1
test text line 2
test text line 3
[johngg@ovs276 perl]$ perl -i.BAK -pe 's/$/,$ARGV/;' *.txt
[johngg@ovs276 perl]$ ls
abc.txt      abc.txt.BAK  def.txt      def.txt.BAK
[johngg@ovs276 perl]$ head -99 *
==> abc.txt <==
test text line 1,abc.txt
test text line 2,abc.txt
test text line 3,abc.txt

==> abc.txt.BAK <==
test text line 1
test text line 2
test text line 3

==> def.txt <==
test text line 1,def.txt
test text line 2,def.txt
test text line 3,def.txt

==> def.txt.BAK <==
test text line 1
test text line 2
test text line 3
[johngg@ovs276 perl]$