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use Modern::Perl;
use File::Slurp qw/read_file/;
...
        say "$key2\t$hash2{$key2}\t$key1" if $hash1{$key1} =~ /\b$hash
+2{$key2}\b/;
    }
}

k_b1    val_a1    k_a1
k_b2    val_a2    k_a1
...
k_b4    val_a4    k_a2
k_b5    val_a5    k_a2