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

Gav.