use warnings;
use strict;
use CGI;
use Data::Dumper;

my $cgi = CGI->new( 'a.b.c=3&d.e.f.g=5&x.y=4' );
my %params=$cgi->Vars();
my %hash;
while (my ($param,$value) = each %params ) {
  my $hashref=fill($param,$value);
  my $key=(keys %$hashref )[0];
  $hash{$key}=$hashref->{$key};
}

print Dumper(\%hash);

sub fill {

  my $node=shift;
  my $value=shift;

  my @nodetree=split /\./, $node;
  if (scalar @nodetree == 1) { return { $nodetree[0] => $value } };
  my $trunk=shift @nodetree;
  $node=join ".", @nodetree;
  return { $trunk => fill($node,$value) };
}
__END__
$VAR1 = {
          'a' => {
                   'b' => {
                            'c' => '3'
                          }
                 },
          'd' => {
                   'e' => {
                            'f' => {
                                     'g' => '5'
                                   }
                          }
                 },
          'x' => {
                   'y' => '4'
                 }
        };