$ cat input
foo



bar
$ perl -ne 'print unless /^$/../^$/' input
foo
bar
$ perl -ne 'print if not /^$/../^$/' input
foo
bar
$ perl -ne 'print if ! /^$/../^$/' input
foo

bar
[download]

The unless statement is equivalent to if not, but is different from if ! due to the associativity and precedence rules covered in perlop. A benefit of this behavior allows the reduction of runs of blank lines to a single blank line.

taken from here