This is a common issue that you should be handling pretty much as you have shown, depending on circumstances. Please see Truth and Falsehood | the first paragraph in Declarations (kinda) or better yet, What is true and false in Perl? (update: or the link (third paragraph) in Eily's post).

Update 1: "Truth and Falsehood" has absquatulated, so had to fix some links.

Update 2:

I find that when I have a string that is just "0" ...
...
I need to use
 if (defined($string)) ...

Just that case might arise if you're reading lines/records from a filehandle with readline($filehandle), e.g., if the last line of the file is "0" and is not newline-terminated (i.e., is just "0"/false and not "0\n"/true). That's why the Perl compiler will "optimise" (if that's the right term) a while-loop condition expression like
    while (my $line = <$fh>) { ... }
to
    while (defined(my $line = <$fh>)) { ... }

c:\@Work\Perl\monks>perl -wMstrict -MO=Deparse,-p -le
"my $filename = 'foo';
 open my $fh, '<', $filename or die qq{opening '$filename': $!};
 ;;
 while (my $line = <$fh>) {
   print $line;
   }
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
(my $filename = 'foo');
(open(my $fh, '<', $filename) or die("opening '${filename}': $!"));
while (defined((my $line = <$fh>))) {
    do {
        print($line)
    };
}
-e syntax OK
[download]

See O and B::Deparse. Compiled under Perl version 5.8.9.