You are checking the truth of $next = pop @x rather than the definedness. 0 is false.

use strict;
use warnings;

my @x = (0, 1, 2, 3);
my $next;

my $first = pop @x ;
print "x $#x pop ",$first,"->";
while ( $next = pop @x )
    { print ",",$next }
print "\n";

@x = (0, 1, 2, 3);
$first = pop @x ;
print "x $#x pop ",$first,"->";
while ( defined ($next = pop @x) )
    { print ",",$next }
print "\n";
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