/(\w+):(\w+(?:,\w+)*)(?:,|$)/g does the trick, as seen here:

use Data::Dumper;

$_ = 'foo:a,bar:a,b,c,baz:d';

my %data;

while (/(\w+):(\w+(?:,\w+)*)(?:,|$)/g) {
   my $key = $1;
   my @vals = split(/,/, $2);
   $data{$key} = \@vals;
}

# Alternative
# ===========
#
# foreach (split(/,(?=\w+:)/)) {
#    my ($key, $val) = split(/:/, $_, 2);
#    my @vals = split(/,/, $val);
#    $data{$key} = \@vals;
# }

print(Dumper(\%data));

__END__
output
======
$VAR1 = {
          'foo' => [
                     'a'
                   ],
          'baz' => [
                     'd'
                   ],
          'bar' => [
                     'a',
                     'b',
                     'c'
                   ]
        };
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