It's as if the use was never executed, the use failed but the exception was caught, or if it's not loading the module it should load.

What does
perl -we "use XML::Simple; print $INC{'XML/Simple.pm'}"
give you? A warning or a file name?

If it returns the a file name, can you give us the first 20-50 lines of that file? What follows is the top lines from the newest version, but even as far back 1.06 is very similar

>perl -we "use XML::Simple; print $INC{'XML/Simple.pm'}"
r:/Utils/perl/site/lib/XML/Simple.pm

>type "r:\Utils\perl\site\lib\XML\Simple.pm"
# $Id: Simple.pm,v 1.23 2005/01/29 04:16:10 grantm Exp $

package XML::Simple;

=head1 NAME

XML::Simple - Easy API to maintain XML (esp config files)

=head1 SYNOPSIS

    use XML::Simple;

    my $ref = XMLin([<xml file or string>] [, <options>]);

    my $xml = XMLout($hashref [, <options>]);

Or the object oriented way:

    require XML::Simple;

    my $xs = new XML::Simple(options);

    my $ref = $xs->XMLin([<xml file or string>] [, <options>]);

    my $xml = $xs->XMLout($hashref [, <options>]);

(or see L<"SAX SUPPORT"> for 'the SAX way').

To catch common errors:

    use XML::Simple qw(:strict);

(see L<"STRICT MODE"> for more details).

=cut

# See after __END__ for more POD documentation


# Load essentials here, other modules loaded on demand later

use strict;
use Carp;
require Exporter;


######################################################################
+########
# Define some constants
#

use vars qw($VERSION @ISA @EXPORT @EXPORT_OK $PREFERRED_PARSER);

@ISA               = qw(Exporter);
@EXPORT            = qw(XMLin XMLout);
@EXPORT_OK         = qw(xml_in xml_out);
$VERSION           = '2.14';
$PREFERRED_PARSER  = undef;
[download]

You are going to have to create a small script that recreates this problem. Remove everything from the script that does not have anything to do with XML::Simple.

Most likely during that exercise you will notice that you have found/fixed the problem yourself. If not, then show us that script and we should be able to help. Right now you haven't given us enough info to know exactly what is wrong, so we are left to guess.