Either your perl behaves very differently from mine, or I didn't describe it very well. If I use these two files:

driver.pl

use TestModule;

print "I am exiting now.\n";
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TestModule.pm

package TestModule;

our $x = 0;
$DB::single = 1;
our $y = $x + 1;
our $z = $y ** 3;

sub blah { "blah!" }

1;
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...and run perl -d driver.pl, the debugger immediately stops at the line our $y = $x + 1. Do you get different results?