[CentOS] perl script part eval help please?

Greetings

Im am not a perl programmer yet I am trying to learn and am debugging
someone elses perl run script for use on a centos production box I have and
I came across this

elif [ -z "${SMTP_CDB:-}" ]

Can someone please shed some light on what this eval means in English
please?

im mainly concerned with the stuff between the two " yet I need all the help
I can get

what does that say? Im trying to figure out if there is a typo or something
in the script.

I am not a perl programmer yet im sure I will be before I get done with this
exercise in the next coupla weeks

Thanks and kind regards

 - rh