in reply to Re^2: newlines in regular expressions
in thread newlines in regular expressions

perl -p -e 'BEGIN { undef $/ } s/\n{2,}/\n/g'

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Which can be shortened to: 

perl -p0777e 's/\n{2,}/\n/g'

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See perldoc perlrun for details of perl's -0 option.

In Section Seekers of Perl Wisdom