It is getting deleted. If you print the hash after the loop (well, anytime after the delete), you'll notice it only has two keys.

foreach my $id (sort keys %test) {
   delete $test{13};
}

print(scalar keys %test, "\n");  # 2
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Your code is equivalent to

@anon_list = sort keys %test;
foreach my $id (@anon_list) {
   print "$id :: $test{$id}\n";
   delete $test{13};
}
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The problem is that you are deleting the key from %test, but not from the list over which foreach is iterating (@anon_list). That list is built when the loop is entered, before delete is called.

A solution is to check if the key still exists when you get around to visiting it.

foreach my $id (sort keys %test) {
   next if not exist $test{$id};
   print "$id :: $test{$id}\n";
   delete $test{13};
}
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