The keys as it were are to note that \n\n is a blank line or if you read a file line by line then a blank line will match m/^\s*$/ 

perl -pi.bak -e 's/\n{2,}/\n/' file.txt
perl -pi -e 's/^\s*\z//' file.txt

perl -ne 'print unless m/^\s*$/' file.txt > noblanks.txt

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Update

Updated as per sgifford's comments.

cheers

tachyon

In reply to Re: newlines in regular expressions by tachyon
in thread newlines in regular expressions by gitarwmn

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