use strict;
use warnings;

my %hash = ();
my @order = ();

foreach (<DATA>) {
    chomp;
    my ($h,$d,$txt) = split /,/;
    my $k = join ',', $h, $d;
    push @order, $k     unless exists $hash{$k};
    $hash{$k} .= ' '    if $hash{$k};
    $hash{$k} .= $txt;
}

foreach my $k (@order) {
    print "$k,$hash{$k}\n";
}

__DATA__
H123456,20151209,THIS IS A TEST
H123456,20151209,TO COMBINE ALL
H123456,20151209,MY MATCHING LINES
H123456,20151209,INTO THE FIRST LINE
H123456,20151209,THAT MATCHES.
H654321,20151209,MATCH LINES FOR THIS
H654321,20151209,ACCT INTO THE
H654321,20151209,TOP LINE OF THE ACCT
H432165,20151209,SINGLE LINE FOR THIS ONE

##</code><code>##

H123456,20151209,THIS IS A TEST TO COMBINE ALL MY MATCHING LINES INTO THE FIRST LINE THAT MATCHES.
H654321,20151209,MATCH LINES FOR THIS ACCT INTO THE TOP LINE OF THE ACCT
H432165,20151209,SINGLE LINE FOR THIS ONE