PadWalker is awfully useful.

Yes, it says it's not for production use. Yes, it violates the principle of least astonishment ("I thought private variables stayed private!"). But it still comes in handy sometimes.

Notable modules that use this

There are a LOT of modules that use PadWalker. Some of the more important ones (IMHO) are:

• doing lexicals ⇒ hash, or hash ⇒ lexicals
  • Var::Extract — hash ⇒ lexicals
• change the behavior of eval()
  • Devel::EvalContext — capture a set of lexical variables, and allow them to be reused later
  • Binding — similar to Tcl's uplevel()
• change the behavior of string-interpolation
  • String::Interpolate
  • String::Bash
• runtime tracing, that prints out the variable names and/or values for each line (or stack dumps that do similar)
  • Devel::DumpTrace
  • Deevel::TraceVars
  • Devel::StackTrace::WithLexicals — stack dump
  • Tie::Trace — display every time a variable is assigned to (you can enable this for many different variables at once)
• enhanced versions of Data::Dumper
  • Data::Dumper::Names
  • Data::Rebuilder