My attempts to access the single item have still failed. :(

Fear not, my good monk, for perldsc, perlref, perlreftut are with you. Just drill down one level at a time.

use strict;
use warnings;

use Data::Dumper;
use XML::Simple;

my $xml = q{
            <SuperItem>
             <UserID>1234</UserID>
             <MajorItems>
              <MinorItem>
               <Item>Box</Item>
              </MinorItem>
             </MajorItems>
             <MajorItems>
              <MinorItem>
               <Item>Suitcase</Item>
              </MinorItem>
             </MajorItems>
            </SuperItem>
            };

my $struct = XMLin( $xml, ForceArray => 1 );
dumpit( $struct );

my $MajorItems_aref = $struct->{MajorItems};
dumpit( $MajorItems_aref );

my $SecondMinorItem_href = $struct->{MajorItems}[1];
# or $MajorItems_aref->[1]
dumpit( $SecondMinorItem_href );

my $MinorItem_aref = $struct->{MajorItems}[1]{MinorItem};
# or $SecondMinorItem_href->{MinorItem}
dumpit( $MinorItem_aref );

my $MinorItemFirst_href = $struct->{MajorItems}[1]{MinorItem}[0];
# or $MinorItem_aref->[0]
dumpit( $MinorItemFirst_href );

my $Item_aref = $struct->{MajorItems}[1]{MinorItem}[0]{Item};
# or $MinorItemFirst_href->{Item}
dumpit( $Item_aref );

my $luggage = $struct->{MajorItems}[1]{MinorItem}[0]{Item}[0];
# or $Item_aref->[0]
dumpit( $luggage );

sub dumpit
{
    print Dumper( $_[0] );
}
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My attempts have succeeded. My thanks to you!