Use the file slurp argument switch, '0'

perl -i.bak -0pe 's/\n\n/\n/g' inputtext

Update: I chose zero instead of octal 0777 thinking it would be clearer but GrandFather may have a good point, (a byte having possible values of 0 to 255 and the base 10 value of 0777 is 511). I guess I was thinking also that double zero, 00, is the switch for paragraph mode reading.

In reply to Re: Removing double carriage return by Cristoforo
in thread Removing double carriage return by dragooneye

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