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sub remove_from_dtgs {
    my ($dtg,$file) = @_;
...
    }
    delete $dtgs{$dtg} unless @{$dtgs{$dtg}};
}

sub remove_from_dtgs {
    my ($dtg,$file) = @_;
...
        }
    }
}

open(my $dtg_file, "<", $infile) or die "Unable to open $infile: $!\n"
+;
while(<$dtg_file>) {
...
    delete $dtgs{$dtg}{$file};
    delete $dtgs{$dtg} if !keys %{$dtgs{$dtg}}
}