#!/usr/bin/env perl 
use strict;
use warnings;
use feature 'say';

my @A = ("Red", "Green", "Yellow");
my @B = ("Yellow", "Black","Yellow","Red", "White", "Yellow");

for my $x (@A){
    my @matches;
    for my $y (0 .. $#B){
        push @matches, $y if $B[$y] eq $x;
    }
    say $#matches < 0 ? "$x is missing" : "$x at index " . join ', ', @matches;
}