open(my $dtg_file, "<", $infile) or die "Unable to open $infile: $!\n";
while(<$dtg_file>) {
    chomp;
    my ($dtg,@files) = split /:/;
    $dtgs{$dtg} = \@files;
}
close $dtg_file;

##</code><code>##

sub remove_from_dtgs {
    my ($dtg,$file) = @_;
    my @files = grep {$_ ne $file} @{$dtgs{$dtg}};
    if(@files == 0) {
        delete $dtgs{$dtg};
    } else {
        $dtgs{$dtg} = \@files;
    }
}