The thing that is really flummoxing me here, Eliya, is that so do I, in a one-liner, but that’s not what I am seeing in the larger script.

perl -e 'use strict; use warnings; my $x=0; my $y=0; if ($x eq my $y) { print "meh"; }'
Use of uninitialized value $y in string eq at -e line 1.

(No message is produced, in this one-liner, if use warnings; is omitted, and of course, this is the behavior that I expect to see all the time.)

But the large Perl file compiles with no such warnings at all!   And that is what is throwing me here.

I can accept the notion of the use of my being legitimate here, and I expect the warning-message to appear in every case when use warnings; is used.   This is what I expected here.   But, in this particular instant case, “no warning!”   The end result is that a subtle bug went into a production situation, and the only way that it was found was by line-by-line eyeball inspection in which three out of four people looking at it (“Paris in the the spring” ...) saw only what they expected to see.