Basically the code provides the same functionality as the unix command basename. The code detects whether the invocation contained any path elements (eg $ ./code.pl or $ /home/user/code.pl) and returns the actual filename.

$this_proc = 
       $0                   # Filename of code (as invoked)
           =~               # "Perform" Regular Expression
               /^.+\/       # Matches everything until the last /
                 (.+)$/     # Capture filename (in $1)
           ?                # If Regex was successful
               $1           #   Return the filename
           :                # Otherwise (Else)
               $0;          #   Return the filename as invoked
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