Question about using Crypt::SSLeay::X509

Hello,

I am using LWP::UserAgent  to access https server with a proxy 
certificate and would like to check if the proxy had already expired 
before trying to contact the server.

I found functions  not_before and not_after in Crypt::SSLeay::X509, 
which  seem to be doing exactly what I  need.

However they expect a $cert as an input. How do I created the $cert 
object from my  proxy file?

Thanks for any help.

     Natalia.