use 5.010;
use strict;

sub read_hash
{
	my $data = shift;
	open my $fh, '<', \$data;
	my %hash;
	while (<$fh>)
	{
		chomp;
		my ($key,$value)=split /,/, $_, 2;
		$hash{$key}=$value;
	}
	return \%hash;
}

my $h1 = read_hash(<<'DATA');
k_a1,val_a1,val_a2,val_a3
k_a2,val_a4,val_a5
DATA

my $h2 = read_hash(<<'DATA');
k_b1,val_a1
k_b2,val_a2
k_b3,val_a3
k_b4,val_a4
k_b5,val_a5
DATA

my %tmp;
while (my ($k, $v) = each $h1)
	{ push @{ $tmp{$_} }, $k for split /,/, $v }
while (my ($k, $v) = each $h2)
	{ push @{ $tmp{$_} }, $k for split /,/, $v }
for (values %tmp)
	{ next if @$_ < 2; say join ',', @$_ }