If you don't care about which sets an item belongs to, and there are no duplicates within any set:

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11152381
use warnings;

my %sets = ( A => [1,2,3], B => [3, 4], C => [1, 3, 4] );

my %seen;
my @items = sort grep $seen{$_}++ == 1, map @$_, values %sets;

print "Items in more than one set: @items\n";
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Outputs:

Items in more than one set: 1 3 4
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