With a oneliner:

$ perl -i -ple '$_.="\n" if $. % 4 == 0; close ARGV if eof' -- FILES

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Documentation: Command-line switches in perlrun, $. for the current line number, eof in regards to resetting $., and Multiplicative Operators for the modulo operator.

In reply to Re: put a newline character after every four lines in a file by haukex
in thread put a newline character after every four lines in a file by Anonymous Monk

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