Sorry to bother you, since I notice other people have also been asking
about defined. But the answers to their questions have not helped me to
this point.

This morning I started seeing a vague, unhelpful warning message about an
undefined variable from a program after months of successful use. The
warning occurs in a test line of the form

if ($xx > $yy) {

So I

1. Checked the input data visually.

2. Checked the data again by going through the algorithm,
using the actual data in the input files.

3. Added print statements for $xx and $yy.

Finding nothing wrong, I created the following test program containing the
essential logic, and with a variable deliberately undefined.

==============
use strict;
use warnings;
use diagnostics;

my $xx = 7;
my $yy;

if ($xx > $yy) {
print "xx > yy, so do stuffn";
} else {
print "Do not do stuffn";
}
==============

Running this program, of course, yields a warning that something is
undefined in the line

if ($xx > $yy) {

So then I added what seems to be a plausible test condition, as shown in
this version:

==============
use strict;
use warnings;
use diagnostics;

my $xx = 7;
my $yy;

if ((defined $yy) == 0) {
print "nyy is NOT defined, so Exitn";
exit;
}

if ((defined $xx) == 0) {
print "nxx is NOT defined, so Exitn";
exit;
}

if ($xx > $yy) {
print "xx > yy, so do stuffn";
} else {
print "Do not do stuffn";
}
==============

In the test program, this fixed the problem, as did several other versions
of the tests. So I added the tests to the real program, intending to add
a bunch of print statements before "exit" so I could analyze the
underlying problem. Unfortunately, the tests using "defined $yy" and
"defined $xx" failed in the real program: absolutely no change in
behavior, except that the line number printed by the warning message
increased to allow for the test code. I tried a whole bunch of tests, and
every single one of them failed.

What have I missed?

Thanks,
Walt