use strict;
my %hash1 = (
	A => 5,
	B => 7,
	C => 2,
	D => 8,
);
my %hash2 = (
	1234 => "A,C",
	4567 => "A,B,D",
	5678 => "C",
);

while ( my( $key2, $val2 ) = each %hash2 )
{
	my %keys1;
	@keys1{ split /,/, $val2 } = ();
	my @missing = grep { not exists $keys1{$_} } keys %hash1;
	print "hash2 key '$key2' is missing values '@missing'\n";
}