foreach my $elA (@arA) {
    foreach my $elB (@arB) {
        if ($elA eq $elB) {
            push @{$all{$elA}}, $elB." - from Array B";
        }
    }
    foreach my $elC (@arC) {
        if ($elA eq $elC) {
            push @{$all{$elA}}, $elC ." - from Array C";
        }
    }
}