use strict;
use warnings;
use Data::Dump qw(pp);

my @a = qw(now is the time for all good men);
my %h;
add_val(\%h, 'FOO', @a);

print pp(\%h);

sub add_val {
    my ($hashref, $val, @keylist) = @_;
    die "Expected a list of keys!" unless @keylist;
    die "Expected a hashref!" unless "HASH" eq ref $hashref;

    while (@keylist > 1) {
        my $key = shift @keylist;
        $hashref->{$key} = {} if ! exists $hashref->{$key};
        $hashref = $hashref->{$key};
    }
    $hashref->{shift @keylist} = $val;
}