package Whatever;
my @required = qw[key1 key2 key3];
sub new {
    bless {}, shift;
}
sub init {
    my $self = shift;
    my %attr = @_;
    foreach my $required (@required) {
        die "Required attribute $required missing" 
             unless exists $attr{$required};
    }
    @$self{keys %attr} = values %attr;
    $self;
}

my $obj = Whatever::->new->init(key1=>1, key2=>2, key3=>3);