#!/usr/bin/env perl -l

use strict;
use warnings;

my %reformat;
my $re = qr{^(H\d+,\d+,)(.*)$};

while (<DATA>) {
    chomp;
    /$re/;
    push @{ $reformat{$1} }, $2; 
}

print $_, join ' ', @{ $reformat{$_} } for keys %reformat;

__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.
H432165,20151209,SINGLE LINE FOR THIS ONE
H654321,20151209,MATCH LINES FOR THIS ACCT INTO THE TOP LINE OF THE ACCT