Perhaps the following will assist you:

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

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

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

for my $key2 ( sort keys %hash2 ) {
    for my $key1 ( sort keys %hash1 ) {
        say "$key2\t$hash2{$key2}\t$key1" if $hash1{$key1} =~ /\b$hash
+2{$key2}\b/;
    }
}

[download]

Output:

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

[download]

A regex is used within map to initialize the hashes with key/value pairs. The grep /.,./ helps insure that the regex can match--in case there are any blank lines (or other non-matching lines) in the files. Instead of splitting %hash1's values on commas, a word-boundary match is used to see if a value in %hash2 exists as a value in %hash1, and both keys are printed if so.

Hope this helps!

Update: Output modified based upon OP's reply to tobyink.

In reply to Re: how to loop through hash tables more efficiently by Kenosis
in thread updated_again: how to loop through hash tables more efficiently by lrl1997

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