Re: Variable being clobbered by a seemingly unrelated while loop

[blindluke]

First of all, your code runs with strict and warnings enabled, no need to comment them out.

The magic $_ variable is an alias, not a copy. This can be useful when you are iterating over an array (with for (@array)), and modifying $_ - you will see that the array elements will change.

In your code, the $runlvl variable is clobbered when the while loop starts. You can test this by changing it to a read only value:

for ('23') {
  /2/ and process($0);
  /3/ and finalize();
}

Now, even without strict and warnings, you will see:

Modification of a read-only value attempted at ./test.pl line 9.

...which corresponds to the beginning of the while loop. The easiest solution would be to change the $_ to a named alias, as in:

for my $switch ($runlvl) {
  $switch =~ /2/ and process($0);
  $switch =~ /3/ and finalize();
}

There is a very good explanation of the default scalar variable, and the behavior that you are experiencing, in the Modern Perl book (here), which says:

As English gets confusing when you have too many pronouns and antecedents, so does Perl when you mix explicit and implicit uses of $_. If you use it in multiple places, one piece of code may silently override the value expected by another piece of code. For example, if one function uses $_ and you call it from another function which uses $_, the callee may clobber the caller's value.

regards,
Luke Jefferson