#! perl -slw
use strict;
use Data::Dump qw[ pp ];

my %hash1 = (
    'key1', 'val1,val2',
    'key2', 'val3'
);

my %hash2=(
    'val1','val5,val6,val7',
    'val2','val8,val9',
    'val3','val3'
);

my %newhash = map {
    $_ => join ',', @hash2{ split ',', $hash1{ $_ } };
} keys %hash1;

pp \%newhash;
__END__
C:\test>junk
{ key1 => "val5,val6,val7,val8,val9", key2 => "val3" }