use Data::Dumper;
$Data::Dumper::Indent = 0;

my $aref1 = [
              'D 4',
              'E 6'
              ];
my $aref2 = [
             'A 0',
             'B 1',
             'C 2',
             'A 3',
             'B 4'
             ];

my $slen = 5;

concat($aref1,$slen);  # This already give correct result
concat($aref2,$slen);  # This still incorrect

sub concat
{
    my  ($aref,$slen)= @_;

    my @concat;
    foreach  my $i ( 1 ..  $#{$aref} )
    {
        my @out =  split (" ", $aref->[$i-1]);
        my @in  =  split (" ", $aref->[$i]);

        # value to be put in (intercepted)
        my $val = $in[1]-$out[1]-$slen;     

        push @concat, ($out[0],$val,$in[0]);
    }

    print Dumper \@concat ;

    return ;
}