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
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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.