在Perl中连接2个哈希时,我得到一个额外的元素"undef"

my %hash1 = (key1 => "value1", key2 => undef);
my %hash2 = (key3 => "value3", key4 => "value4");

# 检查哈希中的值是否为未定义，并将其转换为空字符串
foreach my $key (keys %hash1) {
    $hash1{$key} = "" unless defined $hash1{$key};
}
foreach my $key (keys %hash2) {
    $hash2{$key} = "" unless defined $hash2{$key};
}

# 连接哈希
my %combined_hash = (%hash1, %hash2);