#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd /;
for my $many ( 1..4 ){
        my $s = "a\n".("\n" x $many )."\nb";
    dd( $s );
    while( $s =~ m{^(\s*?)$}gm ){
        my $pos = pos( $s );
        my $one = defined $1 ? $1 : '';
        my $len = length $one;
        my $start = $-[0]; # @-
        my $lend  = $+[0]; # @+
        print "s($start)e($lend)pos($pos)len($len) ";
        dd(
            ( "".substr $s, 0, $pos - $len),
            ( "".substr $s, $pos-$len, $len ),
            ( "".substr $s, $pos  )
        );
    }
}
__END__
"a\n\n\nb"
s(2)e(2)pos(2)len(0) ("a\n", "", "\n\nb")
s(2)e(3)pos(3)len(1) ("a\n", "\n", "\nb")
s(3)e(3)pos(3)len(0) ("a\n\n", "", "\nb")
"a\n\n\n\nb"
s(2)e(2)pos(2)len(0) ("a\n", "", "\n\n\nb")
s(2)e(3)pos(3)len(1) ("a\n", "\n", "\n\nb")
s(3)e(3)pos(3)len(0) ("a\n\n", "", "\n\nb")
s(3)e(4)pos(4)len(1) ("a\n\n", "\n", "\nb")
s(4)e(4)pos(4)len(0) ("a\n\n\n", "", "\nb")
"a\n\n\n\n\nb"
s(2)e(2)pos(2)len(0) ("a\n", "", "\n\n\n\nb")
s(2)e(3)pos(3)len(1) ("a\n", "\n", "\n\n\nb")
s(3)e(3)pos(3)len(0) ("a\n\n", "", "\n\n\nb")
s(3)e(4)pos(4)len(1) ("a\n\n", "\n", "\n\nb")
s(4)e(4)pos(4)len(0) ("a\n\n\n", "", "\n\nb")
s(4)e(5)pos(5)len(1) ("a\n\n\n", "\n", "\nb")
s(5)e(5)pos(5)len(0) ("a\n\n\n\n", "", "\nb")
"a\n\n\n\n\n\nb"
s(2)e(2)pos(2)len(0) ("a\n", "", "\n\n\n\n\nb")
s(2)e(3)pos(3)len(1) ("a\n", "\n", "\n\n\n\nb")
s(3)e(3)pos(3)len(0) ("a\n\n", "", "\n\n\n\nb")
s(3)e(4)pos(4)len(1) ("a\n\n", "\n", "\n\n\nb")
s(4)e(4)pos(4)len(0) ("a\n\n\n", "", "\n\n\nb")
s(4)e(5)pos(5)len(1) ("a\n\n\n", "\n", "\n\nb")
s(5)e(5)pos(5)len(0) ("a\n\n\n\n", "", "\n\nb")
s(5)e(6)pos(6)len(1) ("a\n\n\n\n", "\n", "\nb")
s(6)e(6)pos(6)len(0) ("a\n\n\n\n\n", "", "\nb")