open my $file, "<" , "Filename" or die "$!";
my @buf;

while (<$file>){
   chomp;
   s/^\s*//; # Drop leading blanks
   s/\s*$//; # Drop trailing blanks
   if (0 == $. % 4){
     # Processes the contents of @buf
     # it appears that something like this is what you need:
     print join(" ",@buf, $_), "\n";
     @buf=(); # Reset it
   }else{
     push @buf, $_;
   }
}
close $file;
@buf and print @buf; # In case there is left-over stuff.
[download]

Thanks NetWallah for the reply..

G'day perl_req,

Welcome to the monastery.

It would appear you're not really across how to open files in Perl. Take a look at open. Note how your code looks nothing like the examples given there. Compare with NetWallah's code (above) which does follow the documented format. (You're also closing the wrong filehandle - see close.)

Perl will tell you about various problems with your code if you ask it to. Add use strict; and use warnings; near the top of your code. If you find the messages it produces are a little hard to understand, also add use diagnostics; for longer, more descriptive explanations. (See: strict, warnings and diagnostics.)

Thanks for providing sample input. Unfortunately, you've put each line in its own HTML paragraph (<p>...</p>) which converts all strings of whitespace (spaces, tabs, newlines) to a single space. That's a lot of extra work for you but of no benefit to us: we can't see where the extra whitespace you've described occurs ("... output which is distorted ..."). For future reference, put the whole block of input (output, etc.) within <code>...</code> tags as you did with your code listing: less work for you, more useful for us.

Here's a solution which produces your desired output. I've added arbitrary tabs to the input to simulate the problem which I can't see (as just described).

#!/usr/bin/env perl

use strict;
use warnings;

my $lines_per_block = 4;
my $wanted_id = 'id2';

while (my $joined_block = join_block_of_lines($lines_per_block)) {
    next unless $joined_block =~ /^$wanted_id/;
    print $joined_block, "\n";
}

sub join_block_of_lines {
    my ($lines_per_block) = @_;

    my @block_lines;

    for (1 .. $lines_per_block) {
        my $line = <DATA>;
        return '' unless defined $line;
        $line =~ s/\s*\n$//;
        push @block_lines, $line;
    }

    my $joined_line = join ' ' => @block_lines;

    $joined_line =~ s/\s*\t\s*/ /g;
    $joined_line =~ s/^(\S+)\s+(:)\s+/$1$2/;

    return $joined_line;
}

__DATA__
    id1 : /a text lies here/ 
add name: new add1
ph  no: new phone1
country: new    country1
id2 : /some  other text lies here/ 
add name:   new add2
ph no: new  phone2
    country: new country2
[download]

Output:

$ pm_join_block_of_lines.pl
id2:/some other text lies here/ add name: new add2 ph no: new phone2 c
+ountry: new country2
[download]

Hi Ken, Thank you so much for your reply. I have been trying couple of things but I am stuck at one of them. I will post the question soon if I am not able to solve it myself first. Thanks