use Modern::Perl;
use File::Slurp qw/read_file/;

my ( $hash1, $hash2 ) = @ARGV or die $!;

my %hash1 = map { /(.+?),(.+)/; $1 => $2 } grep /.,./, read_file $hash1;
my %hash2 = map { /(.+?),(.+)/; $1 => $2 } grep /.,./, read_file $hash2;

for my $key2 ( sort keys %hash2 ) {
	for my $key1 ( sort keys %hash1 ) {
		say "$key2\t$hash2{$key2}\t$key1" if $hash1{$key1} =~ /\b$hash2{$key2}\b/;
	}
}

##</code><code>##

k_b1	val_a1	k_a1
k_b2	val_a2	k_a1
k_b3	val_a3	k_a1
k_b4	val_a4	k_a2
k_b5	val_a5	k_a2