I can set a breakpoint during the global compile phase in 5.18.2.

$ perlbrew use 5.18.2t

$ perl -d -E'
   BEGIN {
      say ${^GLOBAL_PHASE};
      $DB::single = 1;
      say "Hi";
   }
'

Loading DB routines from perl5db.pl version 1.39_10
Editor support available.

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

START
main::CODE(0x16bda48)(-e:5):          say "Hi";
  DB<1> r
Hi
void context return from CODE(0x16bda48)
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> q
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