c:\@Work\Perl\monks>perl -wMstrict -le
"my $input = qq{line1\nline2\n\nline4\n\n\nline7\nline8\n};
 print qq{[[$input]]};
 ;;
 $input =~ s{ ^ \s* \n }{}xmsg;
 print qq{<<$input>>};
"
[[line1
line2

line4


line7
line8
]]
<<line1
line2
line4
line7
line8
>>