• The prints aren't in a loop, so why would it print before/after every line?

  perl -e 'while (<>) { print "XXX\n" ;print; print "XXX" }'
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  Or use -n

  perl -ne 'print "XXX\n" ;print; print "XXX"'
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  Or even -p

  perl -pe '$_ = "XXX\n" . $_ . "XXX"'
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• Then there's the useless use of cat. cat filename is a waste of resource. Use "<" instead.

  perl -e'...' < /tmp/file
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  Or since <> (including -n and -p) treats the arguments as files to read, just

  perl -e'...' /tmp/file
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All together:

perl -pe '$_ = "XXX\n" . $_ . "XXX"' /tmp/file
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