If you find XML::LibXML too verbose, you might like its wrapper XML::XSH2:

#!/usr/bin/perl
use warnings;
use strict;

use XML::XSH2;

$XML::XSH2::Map::file    = '1.xml';
$XML::XSH2::Map::project = 'Release DLL|Win32';
$XML::XSH2::Map::old     = 'ReleaseDLL';
$XML::XSH2::Map::new     = 'ReleaseDLL32';

xsh << 'end ;'
    open $file ;
    register-namespace msb http://schemas.microsoft.com/developer/msbu
+ild/2003 ;
    for //msb:PropertyGroup[contains(@Condition, $project)] {
        for ( ./msb:IntDir | ./msb:OutDir ) {
            set . xsh:subst(text(), $old, $new) ;
        }
    }
    save :b ;
end ;
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One of the features of xsh is you can run it in interactive mode in which it is easy to test your more complicated XPath expressions.