How can I set a working breakpoint to a constant expression? - perl

Question

I have Perl code that uses a constant with an initializing block like this:
use constant C => map {
...;
} (0..255);
When I try to set a breakpoint at the ...; line, it does not work, meaning: I can set the breakpoint, but the debugger does not stop there.
I tried:
Start the program with the debugger (perl -d program.pl)
Set the breakpoint in the debugger (b 2)
Reload using R, then run (r) the program
But still the debugger did not stop at the line, just as if I had no breakpoint set.
My Perl is not the latest; it's 5.18.2, just in case it matters...

Answer 1

You are trying to put a break point in a use block.
A use block is in effect a BEGIN block with a require in it.
The Perl debugger by default does not stop in compile phase.
However you can force the Perl debugger into single step mode inside a BEGIN block by setting the variable $DB::single to 1
See Debugging Compile-Time Statements in perldoc perldebug
If you change your code to
use constant C => map {
$DB::single = 1;
...;
} (0..255);
The Perl debugger will stop in the use statement.

Answer 2

You can avoid altering your code if you create a simple module like this (concept originated here):
package StopBegin;
BEGIN {
$DB::single=1;
}
1;
Then, run your code as
perl -I./ -MStopBegin -d test.pl
Pertinent Answer (previous, not-so-pertinent answer is below this one)
If test.pl looks like this:
use constant C => {
map {;
"C$_" => $_;
} 0 .. 255
};
here's what the debug interaction looks like:
% perl -I./ -MStopBegin -d test.pl
Loading DB routines from perl5db.pl version 1.53
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
StopBegin::CODE(0x55db6287dac0)(StopBegin.pm:8):
8: 1;
DB<1> s
main::CODE(0x55db6287db38)(test.pl:5):
5: };
DB<1> -
1 use constant C => {
2: map {;
3: "C$_" => $_;
4 } 0 .. 255
5==> };
DB<2> b 3
DB<3> c
main::CODE(0x55db6287db38)(test.pl:3):
3: "C$_" => $_;
DB<3>
Note the use of the breakpoint to stop inside the map.
Previous, Not-So-Pertinent Answer
If test.pl looks like this:
my $foo;
BEGIN {
$foo = 1;
};
here's what the debug interaction looks like:
% perl -I./ -MStopBegin -d test.pl
Loading DB routines from perl5db.pl version 1.53
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
StopBegin::CODE(0x5567e3d79a80)(StopBegin.pm:8):
8: 1;
DB<1> s
main::CODE(0x5567e40f0db0)(test.pl:4):
4: $foo = 1;
DB<1> s
main::(test.pl:1): my $foo;
DB<1> s
Debugged program terminated. Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
DB<1>
Note the use of the s command to advance, otherwise it'll skip over the BEGIN block in test.pl