#                col1 col2  index  
                       
my $sample1 =  ['str1 str2  1',
                'str3 str4  2',
                'str5 str6  3',
                'str7 str8  4',
                'str9 str10 5'];

my $sample2  = ['str1 str2 0',
                'str3 str4 4'];

my $sample3  = ['str1 str2 3',
                'str3 str4 4'];


##</code><code>##

$result_spl1 = [
                'str1',
                'str3',
                'str5',
                'str7',
                'str9',
               'str10'
               ];

$result_spl2 = [
                'str1',
                'str2',
                'str3',
                'str4',
               ];

$result_spl3 = [
                'str1',
                'str3',
                'str4',
               ];



##</code><code>##

use Data::Dumper;

get_column($sample1);

sub get_column
{
    my  $ar = shift;

    my $all;
    foreach  my $str ( @{$ar}  )
    {
      my @ar = (split " ",$str)[0,1,2];
      push @$all, [ @ar ];
    }


      my @clean;
      my $flag = $all->[0][2]+1;

      my $idx;
      my $diff;
      push @clean, $all->[0][0],$all->[0][1];

      foreach  my $i ( 1 .. @$all-1  )
      {
          $idx = $all->[$i][2];
          $diff = abs($idx-$flag);

          if ( $diff == 1  )
          {
            push @clean, $all->[$i][1];
          }
          else
          {
            push @clean, $all->[$i][0],$all->[$i][1];
           }

          $flag = $idx;
      }
          print Dumper \@clean ;
    return ;
}