It is getting stranger...

Your code does not work for me as well, however this does:

use strict;
use warnings;

my ($x, $y) = (1, 0);

if ($x =~ /whatever/)
{
    $y or last;

    print "Continue with the if clause\n";
}

print "Do something else\n";
[download]

Probably in your case the optimizer is too smart ...

I am using 5.18.1 on Debian.

UPDATE: I was wrong. If you change the regex to !~ (so that the condition is true) you get the same error.