$ cat a.pl
#! /usr/bin/perl
use strict;
use warnings;

use CGI;
use XML::Parser::Expat;

sub foo {
    my $r = shift;

    # Kuerbis' Vorschlag
    if ( ref $r ) {
        printf "1. %s(0x%x)\n", ref($r), $r;
    }

    # einfache Ausgabe
    printf "2. %s\n", $r;
}

# check different references and objects
for my $e ( [], {}, CGI->new, XML::Parser::Expat->new ) {
    foo($e);
}

__END__


$ perl a.pl
1. ARRAY(0x94d78c4)
2. ARRAY(0x94d78c4)
1. HASH(0x9525064)
2. HASH(0x9525064)
1. CGI(0x94f05fc)
2. CGI=HASH(0x94f05fc)
1. XML::Parser::Expat(0x95284ec)
2. XML::Parser::Expat=HASH(0x95284ec)