"POSIX Character Classes" in perlrecharclass gives the names of Unicode properties that are equivalent to the POSIX character classes, e.g. [[:ascii:]] is \p{ASCII} and so on, and then you can use Unicode::UCD to inspect those properties. So if I understand your question correctly:

use warnings;
use strict;
use Unicode::UCD 'prop_invlist';

my @invlist = prop_invlist("PosixAlpha");
for (my $i = 0; $i < @invlist; $i += 2) {
    my $upper = ($i + 1) < @invlist ? $invlist[$i+1] - 1
        : $Unicode::UCD::MAX_CP;
    my @chars = map { chr $_ } $invlist[$i] .. $upper;
    print join(", ", @chars), "\n";
}

__END__

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X
+, Y, Z
a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x
+, y, z
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