In my experience, XML::Simple makes dealing with very simple XML simple but it makes dealing with more complex XML quite difficult. Had you spent two days learning one of the alternatives (I use XML::LibXML almost exclusively these days, but there are other very capable modules that give you good control over both parsing and generation of XML files) you could probably have solved your problem and learned a more powerful tool for future work.

In summary, my suggestion is to use XML::Simple if it does what you need by default or you can tweak it in a few minutes. Otherwise, invest in learning a more general/powerful tool.

Update:

As an example of one way to get your hash with XML::LibXML:

use strict;
use warnings;
use XML::LibXML;
use Data::Dumper;

my $xml = do { local $/; <DATA> };
my $dom = XML::LibXML->load_xml(string => $xml);

my %vendors = map {
       $_->getAttribute('vendorUniqueKey') => $_->getAttribute('name')
    } @{$dom->getElementsByTagName('Vendor')};


print Dumper(\%vendors);

__DATA__
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Import format="3.0" fileId="54630000-3141-1404-4310-7F0000014030" fil
+eCreationDateTime="2012-07-23T06:04:31.415" catalogSource="TLCMz">
    <Catalog>
        <Vendors>
          <Vendor name="Allen Systems" vendorUniqueKey="ALLENGRP"/>
          <Vendor name="Aonix North America" vendorUniqueKey="AONIX"/>
          <Vendor name="Beta Systems Software" vendorUniqueKey="BETASY
+S"/>
          <Vendor name="BMC Software" vendorUniqueKey="BMC"/>
        </Vendors>
    </Catalog>
</Import>
[download]

gives

$VAR1 = {
          'BETASYS' => 'Beta Systems Software',
          'BMC' => 'BMC Software',
          'ALLENGRP' => 'Allen Systems',
          'AONIX' => 'Aonix North America'
        };
[download]

That's simple enough. The problem is, reading all the XML::LibXML docs (there are many) to find out how to do this isn't so simple.