Minor tweak to allow reading of a line containing "0" :

   while (defined(my $line = <$in>)) {
        chomp $line;  # Zap trailing "\n" efficiently 
        print $out $line,"\r";
    }
[download]

Not required:

#!/usr/bin/perl

use strict;
use warnings;
use autodie;

open my $out,'>:raw',"file.tmp";
print $out "a line\na second line\n0";
close $out;

open my $in,'<:raw',"file.tmp";
while (my $line=<$in>) {
    chomp $line;
    print "read: $line\n";
}
close $in;
[download]

>perl foo.pl
read: a line
read: a second line
read: 0
>
[download]

Any line but the last in a file always contains a trailing newline, so $line is guaranteed to be true for all but the last line, and reading empty lines and lines containing only "0" is no problem for all but the last line:

>perl -E '"0\n" and say "true"'
true
>perl -E '"\n" and say "true"'
true
>perl -E '"0" or say "false"'
false
> perl -E '"" or say "false"'
false
>perl -E 'undef or say "false"'
false
>
[download]

The last line WOULD BE a problem if perl would check for truth. But in the special case of readline a.k.a. <HANDLE>, it checks for definedness, not for truth. Quoting perlfunc:

readline EXPR

readline

Reads from the filehandle whose typeglob is contained in EXPR (or from *ARGV if EXPR is not provided). In scalar context, each call reads and returns the next line until end-of-file is reached, whereupon the subsequent call returns undef. [...]

This is the internal function implementing the <EXPR> operator, but you can use it directly.

[...]If either a readline expression or an explicit assignment of a readline expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.

(Emphasis mine)

Alexander