It's consistent because Perl will generate the code the same way each time you compile it. However, another version of Perl may generate slightly different code for the same sequence.

For example, suppose you have the statement $j = $i++ + $i++;. It's perfectly acceptable for perl to implement it as:

   $temp_1 = $i;
   $temp_2 = $i;
   $j = $temp_1 + $temp_2;
   $i = $i + 1;
   $i = $i + 1;
[download]

   $temp_1 = $i;
   $i = $i + 1;
   $temp_2 = $i;
   $i = $i + 1;
   $j = $temp_1 + $temp_2;
[download]

Your co-worker appears to know which way it's implemented on your perl compiler.

roboticus

What?

When the docs say you can't rely on something, it means that the behaviour isn't part of the public interface. It's liable to change without notice.

In fact, docs are more likely to say that about predictable things, since those are the very behaviours which users are likely to assume are reliable.

This sort of thing is more of a problem when you do too much learning through experimenting. You can't tell whether things you learn via experimentation are defined behaviour or an accidental side-effect of implementation, so you need to check them against the docs before relying on them.