Your question isn't very descriptive. Does this do what you want?
Edited to work on Windows
perl -i.bak -pe"BEGIN {@ARGV = map glob, @ARGV} s/a/b/g; s/c/d/g; s/e/f/g; " My/Files/Directory/*.txt
Update
Unless you have only a few changes to make it is probably best if you put the substitution commands in a file, like this
changes.pl
BEGIN {
@ARGV = map glob(""$_""), @ARGV;
}
s/a/b/g;
s/c/d/g;
s/e/f/g;
Then you can run it on a set of files on the command line like this
perl -i.bak -p changes.pl My/Files/Directory/*.txt
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