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

sub remove_from_dtgs {
    my ($dtg,$file) = @_;
    delete $dtgs{$dtg}{$file};
    delete $dtgs{$dtg} if !keys %{$dtgs{$dtg}}
}