I'm wondering about the long-term portability of SECOND and THIRD, though, as they seem to rely on the particular way that constant is implemented.

In a nutshell, this is what sucks about constant. It's not really a built-in, but rather more of a hack based on an implementation detail.

I used it on a big project, and I don't think I would do it again. The syntax for using constants in interpolated strings and HERE docs is horrible, and you have to be constantly alert for subtle bugs caused by the problem you desribe here. I think you're better off with globals.

Some of the mod_perl scripts I inherited used loads of constants. I think they suck because a) interpolation doesn't work properly and b) you get an annoying 'redefinition' warning under mod_perl first time you run the script

I prefer using a stack of variables with all caps at the top of each script, for example:

my $SMTP = 'blah';
my $EMAIL = 'blah@blah.com'
[download]

All my other variables are all lower case, and while these aren't 'true' constants I don't ever modify them.

Gav.

The use of &THIRD can lead to problems so it isn't a habit I encourage people to get into. The most common way that I see problems with this are typical constants exported from modules where &THIRD can cause a warning if $_[0] contains a non-numeric string. Yes, part of the reason that such a warning is possible is because the default way of making constants in the XS code of modules doesn't actually make constants like use constant does (and I think that is changing).

Anyway, using & without () is usually a bad idea, so I don't encourage its casual use. See (tye)Re: A question of style for more on why.