#!/usr/bin/env perl

use strict;
use warnings;

use Data::Dump;

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

my %hash;

my $key;

for my $element (@array) {
    if ($element =~ /^\D$/) {
        $key = $element;
    }
    else {
        push @{$hash{$key}}, $element;
    }
}

dd \%hash;

##</code><code>##

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