"perldoc perldebug" says:

Debugging compile-time statements

If you have compile-time executable statements (such as code within BEGIN and CHECK blocks or use statements), these will not be stopped by debugger, although requires and INIT blocks will, and compile-time statements can be traced with AutoTrace option set in PERLDB_OPTS). From your own Perl code, however, you can transfer control back to the debugger using the following statement, which is harmless if the debugger is not running:

$DB::single = 1;

So it seems you were close but you need to put that BEGIN block in your source code and not in PERL5DB environment variable. (Though I haven't tried this.)

        - tye (but my friends call me "Tye")