Well, I couldn't say your problem description is crystal clear, but the following may get you started:

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dump;

srand 0;

my @aoa = (
    [map {int rand 3} 1 .. 10],
    [map {int rand 6} 1 .. 10],
    [map {int rand 20} 1 .. 10],
);

my @aof = map {{asFreq (@$_)}} @aoa;

print Data::Dump::dump (\@aof);


sub asFreq {
    my @elements = @_;
    my %freqs;
    
    ++$freqs{$_} for @elements;
    return %freqs;
}
[download]

Prints:

[
  { "0" => 6, "1" => 2, "2" => 2 },
  { "0" => 3, "1" => 2, "2" => 2, "3" => 1, "4" => 1, "5" => 1 },
  { "0" => 1, "1" => 1, "5" => 1, "11" => 1, "12" => 1, "13" => 1, "15
+" => 1, "17" => 1, "19" => 2 },
]
[download]