In the test code, lines 4 and 5 comprise a substitution operator. With delimiter pairs for the match and replace clauses, the operator allows whitespace between the clauses (with the single ';' terminating the operator on line 5). And, in fact, the code runs in the debugger (and without the debugger) just fine. It's just the debugger that, it seems, isn't parsing the line, and mis-numbering subsequent lines.
I tried again on a Debian Linux, with Perl 5.28.1 and perl5db.pl 1.5.3. It doesn't show the problem. Maybe that's a clue: I can compare perl5db.pl code from the two machines. I'll report back.