In order to make param processing DWIM, CGI stores GET and/or POST params into the same internal variable. Here's the relevant bits from CGI:

# If method is GET or HEAD, fetch the query from
# the environment.
if ($meth=~/^(GET|HEAD)$/) {
  if ($MOD_PERL) {
    $query_string = $self->r->args;
  } else {
    $query_string = $ENV{'QUERY_STRING'} 
      if defined $ENV{'QUERY_STRING'};
    $query_string ||= $ENV{'REDIRECT_QUERY_STRING'}
      if defined $ENV{'REDIRECT_QUERY_STRING'};
  }
  last METHOD;
}

if ($meth eq 'POST') {
  $self->read_from_client(\$query_string,$content_length,0)
    if $content_length > 0;
  # Some people want to have their cake and eat it too!
  # Uncomment this line to have the contents of the query  
  # string
  # APPENDED to the POST data.
  # $query_string .= (length($query_string) ? '&' : '') . 
  #   $ENV{'QUERY_STRING'} if defined $ENV{'QUERY_STRING'};
  last METHOD;
}
[download]

I've always loved that cake comment in CGI