my %arrays = (
    a1 => \@a1,
    a2 => \@a2,
    a3 => \@a3,
);
my %seen;
    for my $key (keys %arrays) {
    my $array = $arrays{ $key };
    for my $item (@$array[ 1 .. $#$array ]) {
        $seen{ $item }->{ $key }++;
    }
}

for my $key (sort keys %seen) {
    my $value = $seen{ $key };
    if (keys %$value > 1) {
        print "multiple $key in @{[keys %$value ]}\n";
    }
}