It's almost a one-liner :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1198743

use strict;
use warnings;
use Data::Dump 'pp';

my @array = ('a', 1, 2, 3, 4, 'b', 6, 7, 8);

my $ref;
my %hash = map { /\D/ ? ($_ => $ref = []) : (push @$ref, $_) x 0 } @ar
+ray;

pp \%hash;
[download]

Outputs:

{ a => [1 .. 4], b => [6, 7, 8] }
[download]