What kind of file is it, actually? Smells like a csv file with multiline fields, in which those lines are separated by ^M. In any case, that smells like an XY problem.

There's no such thing as a DOS line feed - a line feed is "\n" or ASCII 10. Carriage return is "\r" or ^M or ASCII 13. DOS line endings are "\r\n" (or CRLF). Establish what is your line ending proper (possibly "\r\n"), read the lines setting $/ to that line ending (see perlvar), then convert any (multiple) "\r" occurrences as per the specs of the task (which are those?). A few sample lines would be helpful for further advice.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

moo@cow[1]~>which rnfix
rnfix:   aliased to perl -pi.bk -e 's/\r\n?/\n/g'
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I find it very handy, hence the alias, but use with caution! It's fine for text files. It will break binary files. I added a .bk for the example. Take it out if you're sure you know what you're doing. (update, took out the /g, pretty sure that was just a stupid reflex; update, update: put it back, I need to lie down.)

Thanks to all for their replies. Nothing seems to work. I am trying a different approach. if line ends in ^M and the next starts with ^M join lines. Then remove the ^M^M from middle of the line. Any thoughts?

untested:

$/ = "\r\n";
while (<>) {
    if (s/^\r//) {
        $l .= $_;
        $l =~ s/\r\r//g;
        next;
    } else {
        print $l;
        $l = $_;
    }
}
print $l;
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although I don't know what the heck you are wanting to to with what files to what end.

Could you show what you've tried? and perhaps some sample input?

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

use File::Slurp;

my $text = read_file('filename');
$text =~ s/\r\r/\r/g;
print $text;
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Knowing exactly the structure of the file would help. What does od -c filename print out near those blank lines?

If on a *nix box: Use file (Unix) to determine what sort of critter you have.

Use dos2unix, flip or flip: Newline conversion between Unix, Macintosh and MS-DOS ASCII files to convert.

Or see Super Search: newline regex.