my $str = "bla bla bla :

==bla bla

==bla bla


====bla bla bla


....

....

";

$str =~ s/\n\n+/\n/g;
print qq{\$str = "$str";\n};
__END__
$str = "bla bla bla :
==bla bla
==bla bla
====bla bla bla
....
....
";