Kevin's Weblog

I haven’t posted a Hall of Shame entry in a while, and this one adds a new twist. Today, it’s perl itself that gets the blame. (I know, some will say that’s always the case.) This is about when two magics collide.

There are a couple magic tokens that you can use in perl scripts to represent the current context, like __PACKAGE__ and __LINE__. These tokens behave differently from variables, and one example of this is that they don’t interpolate inside strings.

For reasons of backwards compatibility, when perl encounters a bareword that it doesn’t recognize, it ‘auto-quotes’ it (treats it as a string). This is generally considered A Bad Thing and the strict pragma forbids it. Except for when it doesn’t, and that exception is hash keys. So, even with strict, a bareword hash key is magically auto-quoted.

Now, consider this code:

package foo;
use strict;
my %h; 
$h{__PACKAGE__} = "bar"; 
while (my ($k, $v) = each %h) { print "$k => $v" };

I think it’s clear that anyone writing this code expects the output to be

foo => bar

Unfortunately, it’s actually

__PACKAGE__ => bar

What’s happened is that perl has auto-quoted the string before doing the token substitution. In order to get the desired behavior, it’s necessary to resort to atrocities like

$h{__PACKAGE__ . ""} = "bar"; 

or

$h{+__PACKAGE__} = "bar"; 

to get the job done