Hmmm... You are right. I have edited my earlier post to correct the misinformation.

It seems that I had confused Perl, SQL, and Lisp while writing that. As I recall, there is only one undef value in perl, and DBI (see also DBI recipes) maps SQL NULL and Perl undef. Here are the tests I ran before writing that post:

• $ perl -we 'print "T\n" if "foo" eq "foo"'
  T

• $ perl -we 'print "T\n" if undef eq "foo"'
  Use of uninitialized value in string eq at -e line 1.

• $ perl -we 'print "T\n" if undef eq undef'
  Use of uninitialized value in string eq at -e line 1.
  Use of uninitialized value in string eq at -e line 1.
  T

Spotted the missing test yet? Try:

• $ perl -we 'print "T\n" if undef eq ""'
  Use of uninitialized value in string eq at -e line 1.
  T

Since the message specifically mentions use of undef in eq, I had assumed that eq actually sees the undefined value, instead of stringifying it like any other value. Wrong!