The shell command-line equivalent:
    cat data.txt | perl -00pe 's/\\n/,/g'

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 Update: Oops! My mistake. I needed to add "-l" argument and also be careful of how the shell interprets '\n': 
    cat data.txt | perl -l -00pe 's/\n/,/g'

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In reply to Re^2: how to convert new line to "," by repellent
in thread how to convert new line to "," by bh_perl

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