That means to take the environment variable from your environment (a group of variables provided by your OS, command shell, or the program which called this program) and assign it to the Perl variable $audit_trail if it is defined and non-zero. If the environment doesn't have that variable or it's set to zero, then set $audit_trail to zero.

The program in which that line was used a variable called $audit_trail in its computations or control logic (probably just control logic, and probably just about how much data to log). This could have been explicit in the main program or it could be a feature of a module the main program was using.