#!/usr/bin/perl -w
use strict;
use warnings;

$_ = 'foo:a,bar:a,b,c,baz:d,faa:this,is,great';
my @array = m/(\w+:(?:(?>\w+,?)+)(?!:))/g;
print $_,$/ for @array;

__END__
foo:a,
bar:a,b,c,
baz:d,
faa:this,is,great
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update: Re: Double-duty commas is much more elegant