The Syntax that fixed the issue was my $twig= new XML::Twig(TwigRoots => {'oa:ItemID[@agencyRole="Prefix_Number"]' => \&STOCKnumber,
The code you provided produces a warning while using strict that an explicit declaration of package name is required for @agencyrole.
I am still unable to extract the text of the child element though. Now Perl is claiming that my $Item is undefined.