perl -pe'BEGIN{print"__BEGIN__\n"}s/^\s*$/__BEGIN__\n__END__\n/;END{print"__END__\n"}' <file>


##</code><code>##

perl -pe'$e="__END__\n";BEGIN{$b="__BEGIN__\n";print$b}s/^\s*$/$b$e/;END{print$e}' <file>