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    open my $fh, '<$filename' or die "Can't open $filename: $!";
    open my $dat1, '>$output' or die "\nERROR: Cannot create/open $out

    open my $fh, "<$filename" or die "Can't open $filename: $!";
    open my $dat1, ">$output" or die "\nERROR: Cannot create/open $out