I know what you're saying, but I don't believe it matters here.   Try commenting out either Dying(3) or Dying(4) below...

#!/usr/bin/perl -w

# Strict
use strict;
use warnings;

# Assign $x
my $x = 123;

# Next 2 lines prove $x is defined.
defined $x or die "Dying(1) because (defined \$x or die)\n";
defined $x || die "Dying(2) because (defined \$x || die)\n";

# Make $x undefined
undef $x;

# Doesn't matter which of the next 2 lines is commented-out (either wo
+rks fine)
defined $x || die "Dying(3) because (defined \$x || die)\n";
defined $x or die "Dying(4) because (defined \$x or die)\n";
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