A graphical Perl debugger implemented as a web service
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A Perl graphical debugger that uses an HTTP REST interface


Start a program with the debugger

shell> perl -d:hdb yourprogram.pl
Debugger listening on

Tell the debugger to use a different port

shell> perl -d:hdb=port:9876 yourprogram.pl
Debugger listening on

How to use it

Operation of the interface should be straightforward, though there are a few controls that may not be obvious.

Breakpoints and actions

Clicking on a line number will toggle an unconditional breakpoint for that line. Right-clicking on a line number will bring up a form to enter a breakpoint expression and action. When the expression evaluates true, the debugger will stop on that line.

A line number with a red circle is an unconditional breakpoint. A blue circle is a conditional breakpoint. The circle is dimmed if the breakpoint is inactive, and outlined if that line has an action. Actions are executed before the statement on that line is executed, and the action's result is ignored.

The 'b' hotkey will open the Quick Breakpoint dialog that allows setting unconditional breakpoints. Entering a line number will set a breakpoint on that line in the file the debugger is currently stopped in. Several other expressions are accepted: * . - Set a breakpoint on the current line * subName - Stop on the first line of the named sub in the current package * Some::Package::subName - Stop on the first line of the named sub * subName:linenum - Stop on the nth line of the named sub in the current package * Some::Package::subName:linenum - Stop on the nth line of the named sub * file/name.pm:linenum - Stop on a particular line in a file If it doesn't understand the expression, it'll show an alert describing the problem.

Click on the thick border between the code and watch expression panes to slide out the breakpoint list.

Watch Expressions

Click on the "+" to add a new watch expression, or right-click on a variable in the code pane.

Double-click on an existing expression to edit the expression.

For arrays, hashes and globs, click on the blue circle to collapse/expand it

Click on the checkbox to turn the expression into a watchpoint. Execution will stop if the expression's value changes. Watchpoint values are evaluated in list context. Execution will stop if the list's length changes or if any of the first-level elements changes values. It will not recurse further down into data structures.


The current stack is shown to the left of the code pane. The inital program frame, not part of any function, is called "MAIN". Entering into a function will add a new function name to the top of the list, so that the function the debugger is currently stopped in is always at the top of the list. Mousing over the function name will pop up information showing the full name of the function and what line execution has reached in that frame.

Function names are prepended by a sigil indicating their context/wantarray-ness. String eval frames are represented as "eval".

Clicking on the yellow bar at the top of a code pane will scroll the code to show the currently executing line in that frame.

Mouseover variables

Resting the pointer on a Perl variable in the code pane will show its value. When looking at a stack frame other than the most recent, it will show the value from that stack frame.

Child Processes

If the debugged program forks, it will pop up a dialog giving the option to "Open" a new debugger window, or "Detach" and allow it to run without stopping.


  • Single step, step over, step out, continue and exit the debugged program
  • Set breakpoints dynamically in the running program
  • Inspect the current position at any level in the call stack
  • Watch variables/expressions
  • Get control when the process exits
  • Hover over a variable to see its value. Works for all stack frames.
  • Get notification of untrapped exceptions, and stop at the exception point
  • Stop execution when execution reaches a different point than a previous run

Pretty much what you would expect from any debugger.

Planned Features

  • Step into an arbitrary expression
  • Restart execution at some prior execution point


The debugger is split into two major parts.

  • Devel::hdb::App - Implements the REST interface. It is a Plack app, and uses HTTP::Server::PSGI as the web server. It's a subclass of Devel::Chitin, which supplies the low-level debuggung facilities.
  • The user interface is HTML and JavaScript code that lives under Devel/hdb/html/. It makes requests to the REST interface.