my $ecdsa = Crypt::OpenSSL::ECDSA::ECDSA_SIG->new();

$ecdsa->set_r( $sig->{r} );
$ecdsa->set_s( $sig->{s} );

my $nid = ???( '1.2.3.4' ); # Extracted from CSR
my $key = Crypt::OpenSSL::EC::EC_KEY::new_by_curve_name( $nid);

my $asn1PublicKey = "....I have this";

???$asn1PublicKey = asn1_to_EC_POINT( $asn1PublicKey ); # How?

Crypt::OpenSSL::EC::EC_KEY::set_public_key( $key,$asn1PublicKey )
     or croak( "Failed to create key" );

my $verify = Crypt::OpenSSL::ECDSA::ECDSA_do_verify( 
    sha256( $certificationRequest ), $ecdsa, $key );

# The equivalent for RSA is trivial:

my $key = Crypt::OpenSSL::RSA->new_public_key( $asn1PublicKey );
$key->use_sha256_hash;
$key->use_pkcs1_padding;
$verify = $key->verify( $certificationRequest, $signature );

# DSA is similar