BTW, AFAICT, split /\t/ on two consecutive tabs wouldn't return undef, but rather '' (the empty string):

$ perl -MData::Dumper -e'@a = split /\t/, "foo\t\tbar"; print Dumper \
+@a'
$VAR1 = [
          'foo',
          '',
          'bar'
        ];
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So the whole "//" thing is moot anyway.  It wouldn't even work:

$ perl -MData::Dumper -e'@a = map $_//"NA", split /\t/, "foo\t\tbar"; 
+print Dumper \@a'
$VAR1 = [
          'foo',
          '',
          'bar'
        ];
[download]

In other words, you probably want:

$ perl -MData::Dumper -e'@a = map $_ ne "" ? $_ : "NA", split /\t/, "f
+oo\t\tbar"; print Dumper \@a'
$VAR1 = [
          'foo',
          'NA',
          'bar'
        ];
[download]