Greetings,

#!/usr/bin/perl -w
use strict;
use DBI;

my $pair1 = shift;
my $pair2 = shift;

my $dbh = DBI->connect("DBI:CSV:")
    or die "Cannot connect: " . $DBI::errstr;
$dbh->{RaiseError} = 1;
my $query1 = "SELECT * FROM $pair1";
my $query2 = "SELECT * FROM $pair2";

my $getpair1 = $dbh->prepare($query1);
my $getpair2 = $dbh->prepare($query2);

my %pair1_hash = do{
    $getpair1->execute();
    while($_ = $getpair1->fetchrow_hashref()){
        $_{$_->{Date}} = $_;
    }
    %_;
};
my %pair2_hash = do{
    $getpair2->execute();
    while($_ = $getpair2->fetchrow_hashref()){
        $_{$_->{Date}} = $_;
    }
    %_;
};
my %combo_hash = do{
    map{
        if(!defined $pair1_hash{$_}){
            $_, $pair2_hash{$_};
        }elsif(!defined $pair2_hash{$_}){
            $_, $pair1_hash{$_};
        }else{
            #computations here to combine the contents of 
            #%pair1_hash and %pair2_hash. Utilize the fact that 
            #the last statement evaluated will be the return value.

            #just as and example

            my %t;
        
            #reference to specific fields within each hash referred to
            #by $pair1_hash{$_} and $pair2_hash respectively
        
            $t{Open} = sprintf("%.2f",($pair1_hash{$_}->{Open} + $pair
+2_hash{$_}->{Open} / 2));
            $t{High} = sprintf("%.2f",($pair1_hash{$_}->{High} + $pair
+2_hash{$_}->{High} / 2));
        
            #So finally we return the key and a reference to 
            #our computed hash.

            $_,\%t;
        }
    }do{
        undef @_{keys %pair1_hash, keys %pair2_hash};
        sort keys %_;
        };
};
[download]

The above is untested but the important parts are the creation of hashes keyed by dates (%pair1_hash & %pair2_hash). With our do block we utilize a hash to get all unique values from the list of keys of both hashes, in effect giving us all the unique dated between both hashes. Then with our map statement we take advantage of the fact that the last statement evaluated is the return value so we do some return logic with our if/elsif/else construct. This works because the key must be present in at least one of the hashes that contributed to the list from our do block. I did a sprintf on the average of two of the example columns from your posting above but you will need to do whichever calculations make sense for your purposes. The strategy though is to construct an intermediate hash to hold your computed values then return a reference to it under the key (in effect date) that our map is currently set to, from the list returned from our do block. whew!
Is that what you are looking for?

-InjunJoel