Well, to understand recursion, you first need... to understand recursion. ;-)
More seriously, various useful and valuable suggestions have been made by others above, like using the Devel::Trace module or adding useful print statements to follow what is going on. Another way to try to understand this type of things, that I am using quite frequently when I have trouble to understand how a program works, is to run the program step by step under the Perl debugger. The most useful options to start with may be s, c, n and r to control the flow of your program, p and x to print the value of variables or expressions, and w to add a watch on a variable. Use the h option (help) to better understand what these other options do. Or type perldoc perldebug for more help on the debugger.