The Perl debugger also allows you to define a variety of triggers within your script; these triggers - breakpoints, watch-expressions and actions - come in handy when you need to keep an eye on the values of different variables, since they can be set to automatically notify you in the event of a change.

Breakpoints can be set with the "b" command, which may be followed with either a subroutine name or line number. The following line sets a breakpoint at the first line of the subroutine install_driver() of the DBI
module:

DB<14> b DBI::install_driver
DB<15> L
/usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi/DBI.pm:
576: my $class = shift;
break if (1)

When the debugger reaches this line, it will halt and wait for a new command.

DB<15> n
main::(mailer.pl:12): my $id = $ARGV[0];
DB<15> n
main::(mailer.pl:16): my $dbh =
DBI->connect("DBI:mysql:database=db198;host=localhost", "root",
DBI->"secret",
{'RaiseError' => 1}) or die ("Cannot connect to database");
DB<15> n DBI::install_driver(/usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi
/DBI.pm:576):
576: my $class = shift;
DB<15>

You can set a one-time breakpoint with the "c" command; this command continues executing the script until the breakpoint is reached, at which point it stops and waits for a command.

DB<1> c 19
main::(mailer.pl:19): my $sendmail = "/usr/sbin/sendmail -t";

You can even place a breakpoint within your Perl code, simply by adding the line

  $DB::single=1;

  

or

  $DB::single=2;

  

at the appropriate point in your script. When the debugger encounters this statement while running the script, it will automatically halt and wait for a new command.

You can delete breakpoints with the "B" command, which needs either a line number

DB<8> B 19

or the * wildcard to delete all breakpoints.

DB<9> B *
Deleting all breakpoints...