# create default context
my $ctx = Net::SSLeay::CTX_new or die;
Net::SSLeay::CTX_set_cipher_list($ctx, 'ALL');
Net::SSLeay::set_cert_and_key($ctx, 'cert.pem','key.pem') or die;

# create new context for each new hostname
my %hostnames = ();
Net::SSLeay::CTX_set_tlsext_servername_callback(
    $ctx,
    sub {
        my $ssl = shift;
        my $h = Net::SSLeay::get_servername($ssl);
        unless (exists $hostnames{$h}) {
            $hostnames{$h}->{ctx} = Net::SSLeay::CTX_new or die;
            Net::SSLeay::CTX_set_cipher_list($hostnames{$h}->{ctx}, 'ALL');
            $hostnames{$h}->{cert} = ... # generate certificate and
            $hostnames{$h}->{key}  = ... # key based on hostname in $h
                                         # and re-use them in future from hash as below
            Net::SSLeay::set_cert_and_key(
                $hostnames{$h}->{ctx},
                $hostnames{$h}->{cert},
                $hostnames{$h}->{key}
            ) or die;
        }
        Net::SSLeay::set_SSL_CTX($ssl, $hostnames{$h}->{ctx});
    }
);