Your "environment" is the shell from which you run the script. If you're using bash, you would say, "export MYSCRIPT_DEBUG=1" on the command line, for example. Then run the script. On Windows, I think it's "set MYSCRIPT_DEBUG 1", or "setx MYSCRIPT_DEBUG 1".

You could eliminate the "if $ENV{MYSCRIPT_DEBUG};" test altogether, but then you would have to remember to remove the line of code later on, once things are working as they should. This is just a convenient way to turn on and off debugging behavior without altering your code repeatedly (which provides opportunities for creating new bugs).