1. generate a public/private key pair for encryption use

2. went to https://www.samltool.com/encrypt.php and generated an encrypted SAML2 Assertion, using RSA_OAEP_MGF1P for "Encrypted Method for key", "AES128_CBC" for "Encrypted Method for the data" and the public cert from step 1. Saved the output to a file, encrypted_assertion.xml

3. My private key was in PKCS#8 format, so generated a PKCS#1 version of it using openssl rsa -in myenc.key -out myenc1.key (the Perl code I use only accepts PKCS#1 format)

4. from the SAML2 spec I see that the first 128bits of the encrypted data is actually the IV, https://www.w3.org/TR/2002/REC-xmlenc-core-20021210/Overview.html#aes128-cbc (section 5.2.2)

5. using the code from http://stuff-things.net/2007/05/02/encrypting-sensitive-data-with-perl/ to retrieve the encrypted key from the XML (hence the PKCS#1 version of the private key in step 3, to use in the CBC decoding of the data, I came up with the following code (after much trial and error with the Crypt::CBC parameters):

use strict;
use warnings;
use Crypt::OpenSSL::RSA;
use XML::Simple;
use File::Slurp;
use MIME::Base64 qw(decode_base64);
use Convert::PEM;
use Crypt::OpenSSL::AES;
use Crypt::CBC;

my $xml = read_file("encrypted_assertion.xml");
my $pem = Convert::PEM->new(                              Name => 'RSA
+ PRIVATE KEY',
                              ASN  => qq(
                  RSAPrivateKey SEQUENCE {
                      version INTEGER,
                      n INTEGER,
                      e INTEGER,
                      d INTEGER,
                      p INTEGER,
                      q INTEGER,
                      dp INTEGER,
                      dq INTEGER,
                      iqmp INTEGER
                  }
           ));

my $pemkey = $pem->read(Filename => "myenc1.key");
my $key = Crypt::OpenSSL::RSA->new_private_key($pem->encode(Content =>
+ $pemkey));
my $xml_data = XMLin($xml);
my $CipherKey = decode_base64($xml_data->{"dsig:KeyInfo"}{"xenc:Encryp
+tedKey"}{"xenc:CipherData"}{"xenc:CipherValue"});
my $aeskey =  $key->decrypt($CipherKey);
my $CipherData = decode_base64($xml_data->{"xenc:CipherData"}{"xenc:Ci
+pherValue"});
my $cipher = Crypt::CBC->new(-key => $aeskey, 
                             -cipher => "Crypt::OpenSSL::AES", 
                             -iv => substr($CipherData,0,16), 
                             -literal_key => 1, 
                             -header => 'none', 
                             -keysize => length($aeskey));
print $cipher->decrypt ( substr($CipherData ,16) );
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This takes 0.455s to run.

Using perl -e 'print `xmlsec1 --decrypt --privkey-pem myenc.key encrypted_assertion.xml`' takes 0.015s, and outputs no junk.