Help me understand what you're asking. A newline (\n) is a species of whitespace. Do you mean replace each newline with a space character, i.e., " " (\x20)? If so, you can:

s/\n/ /g;
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G'day Abed,

Is this what you're after:

#!/usr/bin/env perl -l

use strict;
use warnings;

my $data_string = do { local $/; <DATA> };

print "Before replacement: '$data_string'";

$data_string =~ y/\n/ /;

print "After replacement: '$data_string'";

__DATA__
Line 1
Line 2
Line 3
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Output:

Before replacement: 'Line 1
Line 2
Line 3
'
After replacement: 'Line 1 Line 2 Line 3 '
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And, if that doesn't do what you want, please heed earlier advice and indicate exactly what it did produce and how that differed from what you wanted.

Thanks alot Ken, you made my day :) I just edited it by adding \r (I'm using Perl on Windows) and it worked ! --Abed.

while (<$fh>) {
    chomp; # Line in $_ no longer has trailing \n
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Thanks for responding. But chomp only trims the new line at the end of the string.

As example not only different OS standards apply but also encoding question can be raised - please fix my comment if I am wrong but (Unicode Standard Annex #14 docs for Unicode 5.1.0) unicode realization try to fix OS new line problem making no differences between CR, LF && CR+LF chars but when see LF+CR (no CR in first place) - think that's 2 newline characters but ASCII interpreter it as one (historically as i understand).

p.s: please give us you input data example + perl code && environment localization information - so we can check at least you situation does not apply to something exotic like: "Tibetan Line Breaking" unicode new line finding algorithm.

It's difficult to know what the OP needs, even after the OP said, "...I read text from file into string...", since the following reads text from a file into a string, but chomp--in this case--will not remove newlines from $record:

use strict;
use warnings;

local $/ = \1024;

open my $fh, '<', 'records.txt' or die $!;

while ( my $record = <$fh> ) {

    # process 1024-byte, fixed-length, newline-containing record
}

close $fh;
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Hi Man, great idea :) but I need to read the text from large number of huge files, so reading line by line may be not efficient.

You probably don't have to worry about that. While your program may be reading line by line (presumably because that is what you need to do in your algorithm), behind the scenes, Perl is actually buffering input and reading larger chunks of data from the disk (possibly 4 or 8 kB at a time, depending on your OS, hardware, etc.). In brief, there is almost no penalty in reading your file line by line. I am using daily Perl programs to read GB or even dozens of GB of data, if it makes sense in the functional context to read data line by line, then just do it.

No, it won't affect efficiency. Although you attempt to "read" line by line, the underlying library is actually reading block by block, and the <> operator parses the line and give you one line each time.