#!/usr/bin/perl --
use strict;
use warnings;
my ( $hash1, $hash2 ) = @ARGV or die "$!";

my $h1 = read_hash( $hash1, 'reverse' );
my $h2 = read_hash($hash2);

use Data::Dump;
dd $h1, $h2;

while ( my ( $key, $val ) = each %$h2 ) {
    if ( exists $h1->{$val} ) {
        print "$key,$h1->{$val}\n";
    }
}

sub read_hash {
    my $fname = shift;
    my $reverse = shift || 0;

    open( my $fh, "<", $fname ) or die "open( < $fname ): $!";
    my %hash = ();
    while ( my $line = <$fh> ) {
        chomp $line;
        my ( $key, @vals ) = split /,/, $line;
        if ( not $reverse ) {
            ( $hash{$key} ) = @vals;
        }
        else {
            @hash{@vals} = ($key) x int(@vals);
        }
    }
    return \%hash;
}

__END__
$ perl it2.pl hash1 hash2
(
  {
    val_a1 => "k_a1",
    val_a2 => "k_a1",
    val_a3 => "k_a1",
    val_a4 => "k_a2",
    val_a5 => "k_a2",
  },
  {
    k_b1 => "val_a1",
    k_b2 => "val_a2",
    k_b3 => "val_a3",
    k_b4 => "val_a4",
    k_b5 => "val_a5",
  },
)
k_b1,k_a1
k_b4,k_a2
k_b5,k_a2
k_b3,k_a1
k_b2,k_a1