Look the breakpoint is set at runtime!

... you are free to code whatever you want to control it.

Like using a sub dbbreak()

sub dbbreak {
   $DB::single = $DB::my_single_breaks_allowed;
}
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would not only make your code more readable and it'll give you full flexibility for future changes.

update

if you make sure that setting $DB::single is the last command in the sub, then your break will happen after returning!

just tested

$DB::debugger_breaks_allowed =1;


sub break_db {
   $DB::single = $DB::debugger_breaks_allowed;
#   print "inside";
}


print "before";

break_db();
print "after"; #<-- breaks here

print "later";
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And grepping your old code for $DB::single for replacement shouldn't be too problematic...