I'm afraid I couldn't make neither b compile nor b postpone work for me, with different attempts to "compile" a new sub.

Here my experiments using eval , a typeglob and use (applying the technique from Re: Debugger demos/snippets)

Not sure what is going wrong ...

Haven't tested breaking AUTOLOAD tho

use strict;
use warnings;
use Data::Dump qw/pp dd/;



BEGIN {
    
    push @DB::typeahead, split /\n/, <<'__DB__';
B *                                     # delete all Breakpoints
# --- these should fail
b testsub1
b testsub2
b testsub3

b postpone testsub1
b compile testsub1

b postpone testsub2
b compile testsub2

b postpone testsub3
b compile testsub3

L                                       # list breakpoints

use lib '.';
use db_demo_postpone qw(testsub3);

L

c
__DB__

    
    # use after debug-commands
    #    use lib '.';
    #    use db_demo_postpone qw(testsub3);
}





eval <<'__CODE__';
sub testsub1 {
   warn "inside testsub1";
}
__CODE__

*testsub2 = sub {
    warn "inside testsub2";
};



warn "before";

testsub1();

warn "after1";

testsub2();

warn "after2";

testsub3();

warn "after3";
[download]

package db_demo_postpone;
use strict;
use warnings;
use Exporter qw(import);

our @EXPORT_OK  = qw(testsub3);


sub testsub3 {
   warn "inside testsub3";
}

warn "testsub3 compiled";

1;
[download]

OUTPUT: perl -d d:/tmp/pm/db_demo_postpone.pl

Current directory is d:/tmp/pm/

Loading DB routines from perl5db.pl version 1.57
Editor support enabled.

Enter h or 'h h' for help, or 'perldoc perldebug' for more help.

auto(-23)  DB<1> B *                                     # delete all 
+Breakpoints
Line *                                     # delete all Breakpoints no
+t breakable.
auto(-22)  DB<2> # --- these should fail

auto(-21)  DB<3> b testsub1
Subroutine main::testsub1 not found.
auto(-20)  DB<4> b testsub2
Subroutine main::testsub2 not found.
auto(-19)  DB<5> b testsub3
Subroutine main::testsub3 not found.
auto(-18)  DB<6> 

auto(-17)  DB<6> b postpone testsub1
auto(-16)  DB<7> b compile testsub1
auto(-15)  DB<8> 

auto(-14)  DB<8> b postpone testsub2
auto(-13)  DB<9> b compile testsub2
auto(-12)  DB<10> 

auto(-11)  DB<10> b postpone testsub3
auto(-10)  DB<11> b compile testsub3
auto(-9)  DB<12> 

auto(-8)  DB<12> L                                       # list breakp
+oints
Postponed breakpoints in subroutines:
 main::testsub3    compile
 main::testsub1    compile
 main::testsub2    compile
auto(-7)  DB<13> 

auto(-6)  DB<13> use lib '.';

auto(-5)  DB<14> use db_demo_postpone qw(testsub3);
testsub3 compiled at db_demo_postpone.pm line 13.
 at db_demo_postpone.pm line 13.
    require db_demo_postpone.pm called at (eval 17)[c:/Strawberry/perl
+/lib/perl5db.pl:738] line 2
    main::BEGIN() called at db_demo_postpone.pm line 0
    eval {...} called at db_demo_postpone.pm line 0
    eval 'no strict; ($@, $!, $^E, $,, $/, $\\, $^W) = @DB::saved;pack
+age main; $^D = $^D | $DB::db_stop;
use db_demo_postpone qw(testsub3);;
' called at c:/Strawberry/perl/lib/perl5db.pl line 738
    DB::eval called at c:/Strawberry/perl/lib/perl5db.pl line 3138
    DB::DB called at d:/tmp/pm/db_demo_postpone.pl line 45

auto(-4)  DB<15> 

auto(-3)  DB<15> L
Postponed breakpoints in subroutines:
 main::testsub3    compile
 main::testsub1    compile
 main::testsub2    compile
auto(-2)  DB<15> 

auto(-1)  DB<15> c
  DB<15> L
Postponed breakpoints in subroutines:
 main::testsub3    compile
 main::testsub1    compile
 main::testsub2    compile
  DB<15> c
before at d:/tmp/pm/db_demo_postpone.pl line 57.
 at d:/tmp/pm/db_demo_postpone.pl line 57.
inside testsub1 at (eval 20)[d:/tmp/pm/db_demo_postpone.pl:45] line 2.
 at (eval 20)[d:/tmp/pm/db_demo_postpone.pl:45] line 2.
    main::testsub1() called at d:/tmp/pm/db_demo_postpone.pl line 59
after1 at d:/tmp/pm/db_demo_postpone.pl line 61.
 at d:/tmp/pm/db_demo_postpone.pl line 61.
inside testsub2 at d:/tmp/pm/db_demo_postpone.pl line 52.
 at d:/tmp/pm/db_demo_postpone.pl line 52.
    main::__ANON__[d:/tmp/pm/db_demo_postpone.pl:53]() called at d:/tm
+p/pm/db_demo_postpone.pl line 63
after2 at d:/tmp/pm/db_demo_postpone.pl line 65.
 at d:/tmp/pm/db_demo_postpone.pl line 65.
inside testsub3 at db_demo_postpone.pm line 10.
 at db_demo_postpone.pm line 10.
    db_demo_postpone::testsub3() called at d:/tmp/pm/db_demo_postpone.
+pl line 67
after3 at d:/tmp/pm/db_demo_postpone.pl line 69.
 at d:/tmp/pm/db_demo_postpone.pl line 69.
  DB<15>
[download]

As you can see no break happened :/