use autobox::universal qw(type);
use Data::Dumper;
use strict;
use warnings;

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

my %hash = ();
my $key;
for (@array) {
    if (type($_) eq 'STRING') {
        $key=$_;
    }
    else {
         push @{$hash{$key}},$_;
    }
}
warn Data::Dumper->Dump([\%hash],[qw(*hash)]),' ';