We're probably not going to come to an agreement on this.

Note that a lot of people are still using perl 5.005 (constant.pm was first part of 5.6.0), and it wasn't that long ago that Debian and other GNU/Linux distros started installing something a little more recent by default. IIRC, some FreeBSD system scripts won't run on a more recent perl (this may have changed). So there are still a signifcant number of people out there that aren't even on 5.6.

When I'm coding on my main job, I use newer features of the language without a care, because I know we run a more recent version of perl. However, this script was meant for public consumption, which means avoiding newer things if possible (admittedly, I should also have used -w instead of use warnings).

Additional clarity can be gained with a simple # Constants comment above the subroutines in question. I didn't do this in the above code, but it wouldn't hurt.

constant.pm was first part of 5.6.0

Not true!

% /usr/bin/perl -v

This is perl, version 5.005_03 built for i386-freebsd

Copyright 1987-1999, Larry Wall
[...]
% cat c1
#! /usr/bin/perl -wl

use strict;
use constant C => 1;
use constant A => [qw/a b/];
use constant H => {qw/a b/};
print C;
print A;
print A->[0];
print H;
print H->{a};
% /usr/bin/perl c1
1
ARRAY(0x8057c18)
a
HASH(0x8057c3c)
b
[download]

Prior to 5.005_03, constant hashes and arrays didn't work so well, but scalars did. I think scalar constants go all the way back to 5.000 (perhaps you've been tricked by the use 5.006_00 in the source code of later releases)?

FreeBSD 5.2 ships with 5.6.1, and I think 4.9 does as well.