#!/usr/bin/perl

use strict;
use warnings;
use feature qw/say/;

my %parsed = ();

while(<DATA>) {
    chomp;

    my ($key, $values) = split "=", $_, 2;
    my @values         = split ",", $values;

    $parsed{$key} = \@values;
}

foreach my $key (sort keys %parsed) {
    say "$key: ";

    foreach my $value (@{ $parsed{$key} }) {
        say "\t$value";
    }
}

__DATA__
key1=value1
key2=value2,value3,value4,value4,value5,value6,value7,value8,value9,value10
key3=value11,value12

##</code><code>##

$ perl 1119309.pl
key1:
        value1
key2:
        value2
        value3
        value4
        value4
        value5
        value6
        value7
        value8
        value9
        value10
key3:
        value11
        value12
$ 

##</code><code>##

    $parsed{$key} = \@values;

    # ...

    foreach my $value (@{ $parsed{$key} }) {