I have an array which is joined by ';'

you CANT "have an array which is joined by ';'". you could have a string where ';' is a delimiter. So your example code wont even work. did you even try it first?

Anyway if they were valid arrays

for my $i (0..$#next_loop) {
  $next_loop[$i]=$first_loop[$i] unless (defined ($next_loop[$i]) and 
+$next_loop[$i] ne '');
  }
[download]

As huck pointed out, you seem to be starting with CSV-like strings. If that's the case, do yourself an enormous favor and use Text::CSV_XS. One way:

c:\@Work\Perl\monks\Fshah>perl -wMstrict -le
"use Text::CSV_XS;
 use Data::Dump qw(dd);
 ;;
 my $first_record =
   '1668;1733;60;32;3173;0;2517;58221;55764;0;0;0;0;Td;30720;1;9;25;0;
+2; Carry 0';
 my $second_record =
   '1671;1661;0;0;;;0;0;0;0;;;;;;1;9;25;0;2; Carry 1';
 ;;
 my $csv = Text::CSV_XS->new({ sep_char => ';' }) or
   die 'Text::CSV_XS->new{}: ', Text::CSV->error_diag();
 ;;
 $csv->parse($first_record) or die '1st parse failed';
 my @first_fields = $csv->fields;
 dd \@first_fields;
 ;;
 $csv->parse($second_record) or die '2nd parse failed';
 my @second_fields = $csv->fields;
 dd \@second_fields;
 ;;
 die 'number of fields differ' unless @first_fields == @second_fields;
 ;;
 @second_fields =
   map {
     length($second_fields[$_]) != 0
       ? $second_fields[$_]
       : $first_fields [$_]
       }
   0 .. $#first_fields
   ;
 dd \@second_fields;
"
[
  1668,
  1733,
  60,
  32,
  3173,
  0,
  2517,
  58221,
  55764,
  0,
  0,
  0,
  0,
  "Td",
  30720,
  1,
  9,
  25,
  0,
  2,
  " Carry 0",
]

[
  1671,
  1661,
  0,
  0,
  "",
  "",
  0,
  0,
  0,
  0,
  "",
  "",
  "",
  "",
  "",
  1,
  9,
  25,
  0,
  2,
  " Carry 1",
]

[
  1671,
  1661,
  0,
  0,
  3173,
  0,
  0,
  0,
  0,
  0,
  0,
  0,
  0,
  "Td",
  30720,
  1,
  9,
  25,
  0,
  2,
  " Carry 1",
]
[download]

#!perl
use strict;

my @prev = ();
my @data = ();
while (<DATA>){
  chomp;
  my @row = split ';',$_;
  for my $i (0..$#row){
    if ( length ($row[$i]) == 0 ){
      $row[$i] = $prev[$i];
    }
  }
  push @data,\@row;
  @prev = @row; 
}

for (@data){
  print join "\t",@$_,"\n";
}

__DATA__
1668;1733;60;32;3173;0;2517;58221;55764;0;0;0;0;Td;30720;1;9;25;0;2; C
+arry 0;
1671;1661;0;0;;;0;0;0;0;;;;;;1;9;25;0;2; Carry 1;
[download]

Thanks for the reply, I have an array with 100's of these lines how can I perform the same operation over entire array line by line

Do you want to create a new array or overwrite the existing ?