You can specify callback for the parameter and check it there:

my $schema = {
    section => {
        type => 'nested',
        child => {
            key => {
                type => 'string',
                callback => sub { die 'Should be a scalar' if ref $_[1
+] eq 'ARRAY' },
            },
        },
    },
};
[download]

Update: and here's the example:

use warnings;
use strict;
use 5.010;
use Data::Dumper;
use Config::General;
use Config::Validate qw(mkpath);

my $schema = {
    section => {
        type  => 'nested',
        child => { key => { type => 'MyString', }, },
    },
};

my $cfgfile = <<'EOF';
<section>
    key Foo
    key Bar
</section>
EOF

my $config = Config::General->new( -String => $cfgfile );
print Dumper { $config->getall };    ## you can see the array that Con
+fig::General creates
my $validate = Config::Validate->new( schema => $schema );
$validate->add_type( name => 'MyString', validate => \&is_string );
$validate->validate( config => $config );
say "Still alive!";

sub is_string {
    return unless ref $_[1];
    die "Parameter " . mkpath( $_[3] ) . " should be a scalar!";
}
[download]