I have no possibility to test, but I suspect it's related to new features like function signatures.

In a compilation phase they are rewriting the first lines of a functions body.*

Probably your postponed breakpoint happens earlier and points to a line which disappears then.

> I'm not sure where to begin debugging the issue

Otherwise in order to narrow down the cause, you can easily test if it's a problem in the debugger's code.

Perl evaluates (if present) at start up an environment variable pointing to the debugger script to use, normally named perl5db.pl .

It should be possible to run a newer Perl version with an older perl5db.pl, and vice versa.

IIRC the variable is named PERLDB. (to be updated)

HTH! :)

edit

from perlrun#ENVIRONMENT

• PERL5DB

  The command used to load the debugger code. The default is:
  
  BEGIN { require "perl5db.pl" }
  
  The PERL5DB environment variable is only used when Perl is started with a bare -d switch.

footnotes

*) See http://www.effectiveperlprogramming.com/2015/04/use-v5-20-subroutine-signatures/ she Brian is using B::Deparse to analyze the generated code.