I want to authenticate with client SSL certificate. Here is my code 
    my $ua = LWP::UserAgent->new(
        ssl_opts => {
          #SSL_version  => 'SSLv3',
          verify_hostname => 0,
      SSL_cert_file => "./cert.pem",
      SSL_cert_key  => "./key.pem"
    }
    );
    my $res = $ua->get($query);

[download]
I have two questions regarding this.

1. Why is in documentation of IO::Socket::SSL that I can set SSL_version to SSLv3 and when I do so, I get error that this version is not supported?

2. In which format should the key be in? I tried following formats:

a) PEM format = other tools and openssl also works fine with this format so I guess the file is not modified in some weird way.
b) DER format = I converted from PEM format with: openssl rsa -in key.pem -pubout -outform DER -out key.der .... converted with no errors.
c) DER format = I converted from PEM format with: openssl pkcs8 -topk8 -inform PEM -outform DER -in key.pem -out key.der -nocrypt .... converted with no errors.
d) I also tried format that is output of: openssl rsa -in key.pem -text > key1.pem

But every single one yields this error:

DEBUG: .../IO/Socket/SSL.pm:2582: Failed to load key from file (no PEM or DER)
SSL error: 19240: 1 - error:0D08303A:asn1 encoding routines:asn1_template_noexp_d2i:nested asn1 error
SSL error: 19240: 2 - error:0D0680A8:asn1 encoding routines:asn1_check_tlen:wrong tag
SSL error: 19240: 3 - error:0D07803A:asn1 encoding routines:asn1_item_embed_d2i:nested asn1 error
SSL error: 19240: 4 - error:04093004:rsa routines:old_rsa_priv_decode:RSA lib
SSL error: 19240: 5 - error:0D0680A8:asn1 encoding routines:asn1_check_tlen:wrong tag
SSL error: 19240: 6 - error:0D07803A:asn1 encoding routines:asn1_item_embed_d2i:nested asn1 error
SSL error: 19240: 7 - error:140B000D:SSL routines:SSL_CTX_use_PrivateKey_file:ASN1 lib
DEBUG: .../IO/Socket/SSL.pm:2582: global error: Failed to load key from file (no PEM or DER) error:0D0680A8:asn1 encoding routines:asn1_check_tlen:wrong tag

In reply to LWP::UserAgent Client certificate authentication by Anonymous Monk

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