The getAttribute method just takes the name of the attribute for its parameter, not an xpath statement from what I could tell. The way poj did it is likely better but this is a quick fix to your program.

# NOW WORKING
for my $fileline ( $dom->findnodes('/files/file[@name="Image"]/item') 
+)
{
    my ($namenode) = $fileline->findnodes('field[@name="Name"]');
    my ($sizenode) = $fileline->findnodes('field[@name="Size"]');
    say 'Name : ', $namenode->getAttribute('value');
    say 'Size : ', $sizenode->getAttribute('value');
}
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