use strict;
use warnings;

sub test {
open SOURCE, '<', "d:/code/abc.txt" or die $!;
open DEST, '>', "d:/code/def.txt" or die $!;
my $items = qq{};
while (<SOURCE>) {
if (/ID/){
if ($items) {
$items = join(' ', split(/\n/, $items));
print DEST $items, "\n";
$items = qq{};
            }
print DEST $_; # Include the last "\n";
        }
else {
$items .= $_
        }
    }

if ($items) {
$items = join(' ', split(/\n/, $items));
print DEST $items, "\n";
    }

close SOURCE;
close DEST;
}

&test() ;

1;

perl -lpe '$\= /^ID|4$/ ? "\n" : " " ' data.txt