my $quant = $tree_collection{$years}{$trees}{$size} || 0;

##</code><code>##

|| 0

##</code><code>##

 $year_total += $total;
##</code><code>##

#!/usr/bin/perl -w
use strict;
use diagnostics;
use Text::ParseWords;
use Data::Dumper;

my $quantity        = ();
my $total_quantity  = ();
my %tree_collection = ();
my %tree_sizes      = ();

while ( my $line = <DATA> ) {
    my @fields = quotewords( ',', 0, $line );
    my ( $year, $tree, $size, $quantity ) = (@fields)[ 0 .. 4 ];

    $total_quantity += $quantity;
    $tree_collection{$year}{$tree}{$size} += $quantity;
}

# print Dumper ( \%tree_collection );

print "\n";
for my $years ( sort keys %tree_collection ) {
    my %sizeOfTrees = ();
    my @sizeOfTrees = ();
    printf "\n%17s\n", "Planted in year: $years";    #year print
    print "=" x 21, "\n";
    for my $trees ( sort keys %{ $tree_collection{$years} } ) {

        for my $size ( sort keys %{ $tree_collection{$years}{$trees} } ) {
            my @keys = keys %{ $tree_collection{$years}{$trees} };
            @sizeOfTrees{@keys} = ();                # headers (size)
        }
    }
    @sizeOfTrees = sort keys %sizeOfTrees;
    printf "%20s:", "size";

    for (@sizeOfTrees) {
        printf " %8s", $_;                           # headers
    }

    printf "%9s", "Totaal\n";
    print "\n";
    my $total      = ();
    my $year_total = ();
    for my $trees ( sort keys %{ $tree_collection{$years} } ) {
        printf "%20s:", $trees;
        for my $size (@sizeOfTrees) {
            my $quant = $tree_collection{$years}{$trees}{$size};

#          my $quant = $tree_collection{$years}{$trees}{$size} || "-";
    
           printf "%9s", "$quant";

            # { no warnings; $total += $quant; }    # WHY DO I NEED THIS?
        }
        $year_total += $total;
        printf "%5s", "$total";
        $total = 0;
        print "\n";
    }
    printf "\n%21s", "Totaal:";

    my %total_col;

    for my $i ( keys %{ $tree_collection{$years} } ) {
        for my $j ( keys %{ $tree_collection{$years}{$i} } ) {
            $total_col{$j} += $tree_collection{$years}{$i}->{$j};
        }
    }

    # print Dumper ( \%total_col);
    for my $i ( sort keys %total_col ) {
        printf "%9s", "$total_col{$i}";
    }
    printf "%5s", "$year_total";
    print "\n";
}

print "\n";

# year-tree-size-q
print "Total no trees planted: $total_quantity\n";

__DATA__
2010,Oak,10,5
2010,Oak,20,2
2010,Cypress,20,3
2010,Basswood,24,7
2009,Oak,10,4
2009,Oak,25,2
2009,Cypress,20,1
2008,Basswood,25,9
2011,Pine,11,5