my $string = q{
this
is

a
sample
text.


all single blank lines will be removed.

all double, triple, or more


blank lines will not

be touched.
};

$string =~
  s#\n(\n+)#"\n" x (length($1)>1 ? length($1)+1 : length($1))#eg;

print $string;

__END__

this
is
a
sample
text.


all single blank lines will be removed.
all double, triple, or more


blank lines will not
be touched.