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
[download]

Output:

Before replacement: 'Line 1
Line 2
Line 3
'
After replacement: 'Line 1 Line 2 Line 3 '
[download]

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.