Try this, it's probably similar to what CGI.pm does internally to switch modes, or read the module itself. It's pretty much self-documenting. Or maybe look at one of the CGI Lite variants for easier reading. The GET POST detection is done in the %ENV hash.

if ($ENV{'REQUEST_METHOD'} eq "GET") {  
$form_data = $ENV{'QUERY_STRING'};   
 }else {
$form_data = <STDIN>;  
}
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Now to do it with CGI, do this:

#!/usr/bin/perl
use CGI;
my $q = new CGI;
my %param = $q->Vars;

#To check whether the params were sent via POST, do:

my $method = $q->request_method();
print "$method\n";
#which should give either 'POST', 'GET' or 'HEAD'.
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There are alot of cute CGI scripts out there to help in debugging, just google for them. For instance:

#!/usr/bin/perl
use warnings;
# arg_echo.cgi - print all form values
use strict;
use CGI qw/:standard/;

$|=1;
print "Content-type: text/plain\n\n";

foreach my $name ( param() ) {
   print "$name: '", param($name), "'\n";
   }
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