Now, sometimes you do need to change something in the consumer lexical space. So,
exist for this reason too .

Example for the above:

...

And this is essentially the "C"-like '#include "some_file" recipe.

I didn't know that eval() changed existing variables in your program.

And BTW why are you using both "use warnings;" and "perl -w"?

My understanding to this would be that '-w' will deliver the runtime warnings, but the "use warnings" allows you to then create your own,
ie like "warn "\$x=$x";"

I don't think so. I also wondered why Sergei is using both the flag (-w) and the pragma (use warnings;), so we'll see what he says. (The pragma is generally recommended now. For some of the details about how -w and use warnings; differ, see this thread on Stack Overflow.)

In the script, warn is not a warning Sergei created. It's a function built into Perl itself. The difference between warn and print or say is that warn defaults to STDERR, while the others default to STDOUT. I've noticed over the years that Sergei pretty much always uses warn not print for little example scripts like this. I don't know why and haven't seen anyone else do it. It's seems to be a personal preference.

warn also seems to print the line number, which I find annoying. I guess it's good for debugging, though.

To just print to stderr in normal cases, I like:
.

There are two uses of eval. The block form is for exception/error handling. The form Sergei has here, the string form, doesn't exactly "change existing variables." It takes a string and treats it like Perl code. As the docs put it, "the return value of [the expression] is parsed and executed as if it were a little Perl program." It's worth noting that you should not use the string form on any potentially insecure strings (like say, user input). If you ask the user for some string and then eval it and the user puts system("rm *") - as a joke, then you have a problem.

You can turn off the line-number printing by adding a final new line (as with die).

OK, but what is preferred to print error and warning messages: "warn" or "print STDERR"?

For actual warning or error messages, I would use warn. Sergei also likes it for examples and debugging, that's all. There's no right or wrong here, just different preferences. As for "what is preferred", Perl's motto is "there's more than one way to do it", so if you ask five Perl programmers, you will probably get six opinions.

Hey Telemachos - thanks for the correction and further details ... like i said earlier .. still learning

Regarding '-w' - automatic habit since the nineties when there was no "use warnings;".

perldoc -f warn

- the very first sentence with explanations.