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

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