sub flatten {
    my $hr = shift;

    my %result;
    while (my ($k, $v) = each %$hr) {
        if (ref $v eq 'HASH') {
            my $r = flatten($v);
            $result{"$k.$_"} = $r->{$_} for keys %$r;

        } else {
           $result{$k} = $v;
        }
    }

    \%result;
}

print Dumper flatten {
                       foo => {
                           in_foo => 1,
                           bar => 1,
                           baz => { 
                               in_baz => 1
                           }
                       },
                       blah => 1
                     };
__END__

$VAR1 = {
          'foo.bar' => 1,
          'blah' => 1,
          'foo.baz.in_baz' => 1,
          'foo.in_foo' => 1
        };