#!/usr/bin/env perl -l

use strict;
use warnings;

use List::Util qw{first};

my @key_array = 'a' .. 'i';
my @set_array = ( ['a', 'b'],  ['e','f'], ['f','g'] );
my %sets;

for my $set_ref (@set_array, map {[$_]} @key_array) {
    my $key = first { exists $sets{item}{$_} } @$set_ref; 
    my $set_data = defined $key
                 ? $sets{item}{$key}
                 : do { push @{$sets{data}}, {}; $sets{data}[-1] };

    for my $item (@$set_ref) {
        $sets{item}{$item} = $set_data unless exists $sets{item}{$item};
        ++$set_data->{$item};
    }
}

for (sort { @$b <=> @$a } map { [ sort keys %$_ ] } @{$sets{data}}) {
    print join ',' => @$_;
}