my @list_var;
if (ref $giant_hash{key} eq 'ARRAY') {
    for (@{ $giant_hash{key} }) {
        push @list_var, My::Class::Child->new( $_ );
    }
} else {
    push @list_var, My::Class::Child->new( $giant_hash{key} );
}

##</code><code>##

our %giant_hash = Module->whatever();

sub _fill_lists {
    my ($thing, $thing_ref) = @_;

    my ($method , $flag);
    SWITCH: for ($thing) {
        /XXX/ && do {
            $method = \&My::Class::XXX->new;
            last;
        };
        /YYY/ && do {
            $method = \&My::Class::YYY->new;
            last;
        };
        /etc/    && do {$and_so_on; last; };
    }

    if (ref $giant_hash{$thing} eq 'ARRAY') {
        for (@{ $giant_hash{$thing} }) {
            push @{ $thing_ref }, $method->( $_ );
        }
    } else {
        push @{ $thing_ref }, $method->( $giant_hash{$thing} );
    }

}

##</code><code>##

Can't use string ("XXX") as an ARRAY ref while "strict refs" in use at ./testme.pl line 76.