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

##</code><code>##

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

##</code><code>##

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

# In case $ref_to_array was autovivified,
$config->{groups}->{$last_group} = $ref_to_array;

##</code><code>##

my $ref_to_array = $config->{groups}->{$last_group} ||= [];
push @$ref_to_array, $line;