Perl on Win32 Question.

I have inherited a perl script that is supose to check to be sure a system is idle before it does reboots.
The only thing I have not figured out is the part where it checks if the system is really idle, and I have not found anything in the script
that does this check. There is the following but I really don't know what it means.
 

 $arg = shift(@ARGV);     # this is set before the code starts
  
 
 if ($arg =~ /debug/i)       # This is where it is used, it is in a sub.
 

 
does "/debug/i" check to see if a computer is idle? 
if it does is it checking for a duration of idleness?
 
I can post the whole script if needed but the script is really about logging the process.