my($datetime, $data) = /^(\w+-\d+,[\d:.]+),(.*)/;
##</code><code>##

push @{ $data{$datetime} }, $data;
##</code><code>##

foreach my $key (sort keys %data) { ...
##</code><code>##

    print $key, ',', join(',', @{ $data{$key} }), "\n";

##</code><code>##

#! 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";
}

##</code><code>##

Feb-21,19:08:05.2,$GP,48.96,90.92,45.69,$BM,3.89
Feb-21,19:08:06.4,$GP,48.92,90.92,45.70
Feb-21,19:08:07.6,$GP,48.93,90.99,45.66,$BM,6.20
Feb-21,19:08:08.1,$GP,48.92,90.95,45.66
Feb-21,19:08:09.0,$GP,48.85,90.92,45.62,$BM,8.52
Feb-21,19:08:10.8,$GP,48.92,90.94,45.63,$BM,9.68
Feb-21,19:21:20.5,$BM,5.05
Feb-21,19:21:20.8,$BM,7.36