#! perl -w

@ARGV = ('file1.txt', 'file2.txt');
my %data; 
while(<>) {
    my($datetime, $data) = /^(\w+-\d+,[\d:.]+),(.*)/;
    push @{ $data{$datetime} }, $data;
}
foreach my $key (sort keys %data) {
    print $key, ',', join(',', @{ $data{$key} }), "\n";
}