And, they have to be .pem formatted files?

Not necessarily. The documentation says:

Supported file formats are PEM, DER and PKCS#12, where PEM and PKCS#12 can contain the certificate and the chain to use, while DER can only contain a single certificate.

🦛

In reply to Re^3: Issue with LWP loading client certificate by hippo
in thread Issue with LWP loading client certificate by ffrost

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