Aha! Here is the explanation.

Two days ago I was working on this bug report: RT 130445. In the course of that work, I created this hidden file in the original directory:

$ ack 'db\.out' .
.perldb
1:&parse_options("NonStop=0 TTY=db.out");
[download]

So when I ran the perl debugger over any program from that directory, it was silently processing '.perldb' and setting the 'NonStop' option and redirecting legitimate output to a file in that directory called 'db.out'.

Once I deleted '.perldb', the output of perl -d abc.pl returned to normal.

Thank you very much.