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

my $rows = [ ];

while ( <DATA> )
{
    chomp;
    
    my $depth = $_ =~ s/\s{4}//g;
    $depth ||= 0;

    push @$rows, { depth => $depth, key => $_ };
}

my $ref = process( { }, $rows );

print Dumper $ref;

sub process
{
    my ( $ref, $rows ) = @_;
    
    my $current_row = shift @$rows;
    my $next_row    = $rows->[0] // return $ref;
    
    my $current_key   = $current_row->{key};
    my $current_depth = $current_row->{depth};
    my $next_key      = $next_row->{key};
    my $next_depth    = $next_row->{depth};
    
    print "$current_key, $current_depth, $next_key, $next_depth\n";
    
    $ref = $ref->{$current_key} = { };
        
    if ( $current_depth > $next_depth )
    {        
        return $ref;
    }
    elsif ( $current_depth < $next_depth )
    {
        $ref = process( $ref, $rows );
    }
    
    return $ref;
}

__DATA__
one
    two
    three
        four
five
    six