### test1.csv ###
1  aaa  ignored_field
3  ccc ignored_field
4  ddd ignored_field

### test2.csv ###
11 aaa ignored_field
22 bbb ignored_field
44 ddd   ignored_field

### test3.csv ###
333 ccc ignored_field
555 eee ignored_field
666  fff   ignored_field
777  ggg   ignored_field

##</code><code>##

#!/usr/bin/perl
use v5.14;

my %data;

sub cnt_fields {
    my @fields = split "\t", $_[0];
    return scalar @fields;
}

sub gather_file {
    state $count = 0;
    return $count unless @_;

    my ($filename, $d) = @_;
    open my $fh, "<", $filename;
    while (<$fh>) {
        chomp;
        my @field = split;
        my $offset = $count - cnt_fields($d->{$field[1]});
        $d->{$field[1]} .= "0\t" x $offset;
        $d->{$field[1]} .= "$field[0]\t";
    }
    close $fh;

    for my $key (keys %$d) {
        if (cnt_fields($d->{$key}) <= $count) {
            $d->{$key} .= "0\t";
        }
    }
    $count++;
}

for (1..3) {
    gather_file("test$_.csv", \%data);
}

my $header = "Key\t";
for (1..gather_file()) {
    $header .= "f$_\t";
}
say $header;

for my $key (sort keys %data) {
    say "$key\t$data{$key}";
}

##</code><code>##

$ ./report.pl 
Key	f1	f2	f3
aaa	1	11	0	
bbb	0	22	0	
ccc	3	0	333	
ddd	4	44	0	
eee	0	0	555	
fff	0	0	666	
ggg	0	0	777