use Data::Dumper;

my $last_group = "group_z";
my $line = "a line";

my $config = {          # a hash...
    groups => {         # ...of a hash...
        group_z => [    # ...of a list
            2, 3, "Peter", "Pan",
        ],
    },
};

push @{ $config->{groups}->{$last_group} }, $line;

print Dumper $config;