#!/usr/bin/perl
use warnings;
use strict;

use Text::CSV_XS;

my $csv = 'Text::CSV_XS'->new({binary => 1});
open my $CSV, '<', shift or die $!;

while (my $row = $csv->getline($CSV)) {
    $csv->say(\*STDOUT, [ map length ? $_ : 'NULL', @$row ]);
}
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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Note that this approch changes both 1,2,,3,4 and 1,2,"",3,4 to 1,2,NULL,3,4.

The safe way would be

$ cat test.pl

use 5.18.2;
use warnings;
use Text::CSV_XS qw( csv );

csv (in => csv (
        in             => *ARGV,
        blank_is_undef => 1,
        on_in          => sub { $_ //= "NULL" for @{$_[1]}; },
        ),
     );

$ cat test.csv
1,2,,3,4
1,2,"",3,4

$ perl test.pl test.csv
1,2,NULL,3,4
1,2,,3,4
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Enjoy, Have FUN! H.Merijn

awesome

this meets my requirement, all other suggestions are even replacing 0 with NULL.

Could you please suggest me how can i call csv file directly inside the script instead of calling it on command prompt ?

This is what I am running using your input:

#!/usr/bin/perl
#use strict;
use warnings;
use Text::CSV_XS qw( csv);


csv (in => csv (
        in             => *ARGV,
        blank_is_undef => 1,
        on_in          => sub { $_ //= "NULL" for @{$_[1]}; },
        ),
     );
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perl test.pl 1.csv

all i need is to call 1.csv inside test.pl only

perl test.pl

what have you tried so far? I'm not a csv expert but you can do for sure something similar to:

@current_row = map { $_ ? $_ : qq(NULL) } @current_row

you can also do the job in a oneliner

perl -lanF',' -e "print join',',map {$_ ? $_ : 'NULL'} @F " testcsvnul
+l.txt
A,NULL,D,E,F
L,T,NULL,G,Q
C,D,E,NULL,L
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L*

NOT WORKING :

<$ perl -lanF',' -e "print join',',map {$_ ? $_ : 'NULL'} @F " 1.txt
Bareword found where operator expected at -e line 1, near "1.txt"
        (Missing operator before txt?)
Bareword found where operator expected at -e line 1, near "1.txt"
        (Missing operator before txt?)
syntax error at -e line 1, near "1.txt "
Execution of -e aborted due to compilation errors.
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My impression is that Discipulus's
    perl -lanF',' -e "print join',',map {$_ ? $_ : 'NULL'} @F " testcsvnull.txt
is running under Windoze, and your
    <$ perl -lanF',' -e "print join',',map {$_ ? $_ : 'NULL'} @F " 1.txt
is running under *nix. Fix the quoting as appropriate to your OS shell and give it another try.

---

Give a man a fish:  <%-{-{-{-<

I think the advice of choroba and Tux to base your solution on Text::CSV is well taken. But I'm a hopeless regex junky, so here's that approach (requires Perl verion 5.10 or greater):

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 my @records = (
   ',P,Q,R,S',
   'A,,D,E,F',
   'L,T,,G,Q',
   'C,D,E,,L',
   'V,W,X,Y,',
   ' , , , ,',
   ',,,,',
   ',',
   ' , ',
   '',
   ' ',
   );
 ;;
 for my $rec (@records) {
   print qq{'$rec'};
   $rec =~ s{
     (?: \A | ,) \K \s* (?= ,) | (?<= ,) \s* (?= , | \z) | \A \s* \z
     }{NULL}xmsg;
   print qq{'$rec' \n};
   }
"
',P,Q,R,S'
'NULL,P,Q,R,S'

'A,,D,E,F'
'A,NULL,D,E,F'

'L,T,,G,Q'
'L,T,NULL,G,Q'

'C,D,E,,L'
'C,D,E,NULL,L'

'V,W,X,Y,'
'V,W,X,Y,NULL'

' , , , ,'
'NULL,NULL,NULL,NULL,NULL'

',,,,'
'NULL,NULL,NULL,NULL,NULL'

','
'NULL,NULL'

' , '
'NULL,NULL'

''
'NULL'

' '
'NULL'
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