use strict;
use warnings;
use Data::Dumper;

my %hash;
my @row;

while(<DATA>){
  my ($level, $word) = /^(\s*)(\w*)$/;
  $level = length($level)/4;
  $row[$level] = $word;
  $#row = $level;
  
  my $last = \%hash;
  for (0..$#row) {
    $last->{$row[$_]} = $_ == $#row ? undef : {} if not defined $last->{$row[$_]};
    $last = $last->{$row[$_]};
  }
}
print Dumper \%hash;

__DATA__
one
    two
    three
        four
five
    six