This is the perl debugging. If you invoke Perl with the -d switch, your script runs
under the Perl source debugger. This works like an interactive Perl environment,
prompting for debugger commands that let you examine source code, set breakpoints,
get stack backtraces, change the values of variables, etc. This is so convenient
that you often fire up the debugger all by itself just to test out Perl constructs
interactively to see what they do.
