G'day Karim,

Welcome to the Monastery.

Firstly, I concur with ++haukex' response: Text::CSV is the appropriate tool for this task.

"After many attempts at using perl regexes, I am still stuck!"

In some other situation, where you simply want to convert a multi-line string to a single-line string, and preserve line-endings if they exist, you can use this regex:

s/\R(?!\z)//gm
[download]

Here's a quick command line test:

$ perl -E 'my @x = ("a\nb\nc\n", "a\nb\nc", "abc\n", "abc");  for (0 .
+. $#x) { say "$_=|$x[$_]|"; $x[$_] =~ s/\R(?!\z)//gm; say "$_=|$x[$_]
+|" }'
0=|a
b
c
|
0=|abc
|
1=|a
b
c|
1=|abc|
2=|abc
|
2=|abc
|
3=|abc|
3=|abc|
[download]

See also: "perlrebackslash: \R"; "perlre: Lookaround Assertions"; perlre.

Caveat: \R was introduced in v5.10.0: you'll need at least that version of Perl.