This first bit is just setting up 4 arrays with random integer test data.

my @a = (shuffle 1 .. $N)[ 0 .. $E ];
my @b = (shuffle 1 .. $N)[ 0 .. $E ];
my @c = (shuffle 1 .. $N)[ 0 .. $E ];
my @d = (shuffle 1 .. $N)[ 0 .. $E ];
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This bit builds a hash of hashes using the values from the arrays as primary keys and the 'name' of the array that value came from as the secondary key.

my %comp;
$comp{ $_ }{a} = 'a' for @a;
$comp{ $_ }{b} = 'b' for @b;
$comp{ $_ }{c} = 'c' for @c;
$comp{ $_ }{d} = 'd' for @d;
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print a header

print "     : a b c d\n--------------";
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And this bit does the display

printf "%4s : %s\n", $_, 
    join ' ', map{ $_||'-' } @{ $comp{ $_ } }{ 'a'..'d' }
    for sort{$a<=>$b} keys %comp;
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In reverse order

for sort{$a<=>$b} keys %comp;
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For each key (ie. each unique value from the four arrays), in the primary hash, sorted into (in this case ascending numeric order),

@{ $comp{ $_ } }{ 'a'..'d' }
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Take a slice across the hash for this value,

map{ $_||'-' }
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Pass the values through a map to replace undef values by a token ('-') to represent that this value was missing in this array.

join ' ',
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joins the 'found' and 'missing' tokens into a string with some spaces for presentation.

printf "%4s : %s\n", $_,
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and print out the value, and the string showing which arrays it was found in.

Each iteration of that loop gives you one line showing 'this value' appeared/was missing in these arrays. So for my test data you read this line:

   6 : a b - d
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As "The value 6 appeared in array @a @b @d but was missing from @c"