use strict;
use XML::Rules;

my $filter = XML::Rules->new(
    style => 'filter',
    rules => {
        'IntDir,OutDir' => sub {
            my ($tag,$attr,$context,$parents) = @_;
            $attr->{_content} =~ s/\bReleaseDLL\b/ReleaseDLL32/
                if $context->[-1] eq 'PropertyGroup' && $parents->[-1]
+->{Condition} =~ /'Release DLL\|Win32'/;
            return $tag => $attr;
        },
    }
);

$filter->filterfile($inputFilename, $outputFilename);
[download]

Or if you want to be correct according to the namespaces

use strict;
use XML::Rules;

my $filter = XML::Rules->new(
    style => 'filter',
    namespaces => {
        'http://schemas.microsoft.com/developer/msbuild/2003' => 'ms',
        '*' => 'keep'
    },
    rules => {
        'ms:IntDir,ms:OutDir' => sub {
            my ($tag,$attr,$context,$parents) = @_;
            $attr->{_content} =~ s/\bReleaseDLL\b/ReleaseDLL32/
                if $context->[-1] eq 'ms:PropertyGroup' && $parents->[
+-1]->{Condition} =~ /'Release DLL\|Win32'/;
            return $tag => $attr;
        },
    }
);

$filter->filterfile($inputFilename, $outputFilename);
[download]

Not using XML::LibXML though :-)