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

#!/usr/bin/perl

...
    print "read: $line\n";
}
close $in;

>perl foo.pl
read: a line
read: a second line
read: 0
>

>perl -E '"0\n" and say "true"'
true
...
>perl -E 'undef or say "false"'
false
>