# This is the custom debugger:
{ package DB;
  sub DB {}
  sub sub {
    print STDERR "== Entering sub $sub\n";
    &$sub; # Call the sub
    print STDERR "=== Leaving sub $sub\n";
  }
}

# Now some example code to test it with:
sub hello {
  print "Hello, ";
  world();
}

sub world {
  print "World!\n";
}

$|++;   # Turn off buffering, to make the control flow clearer
hello();

##</code><code>##

$ perl d.pl
Hello, World!
$ perl -d d.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

== Entering sub main::hello
Hello, == Entering sub main::world
World!
=== Leaving sub main::world
=== Leaving sub main::hello

##</code><code>##

  BEGIN {$^P = 1}

##</code><code>##

BEGIN {
  return if $^P;  # If the debugger is running, leave well alone
  $^P = 1;        # Enable subroutine tracing
  package DB;
  *sub = sub {
    print STDERR "== Entering sub $sub\n";
    &$sub;
    print STDERR "=== Leaving sub $sub\n";
  };
}