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  find . -type f \! -name '*.bak' -print0 | \
    xargs -0 perl -i.bak -plwe 's/foo/bar/g'

  find . -type f -name '*.html' -print0 | \
    xargs -0 perl -i.bak -plwe 's/foo/bar/g'

  perl -i.bak -plwe 's/foo/bar/g' file1.txt file2.txt file3.txt

  for i in file1.txt file2.txt file3.txt
  do
      mv $i $i.bak
      perl -plwe 's/foo/bar/g' < $i.bak > $i
  done