Usage of defined with Filehandle and while Loop

Question

While reading a book on advanced Perl programming⁽¹⁾, I came across this code:

while (defined($s = <>)) {
    ...

Is there

Answer 1

while($line=){
    chomp($line);
if(***defined*** $line){
    print "SEE:$line\n";
}
}
__DATA__
1
0
3

Try the code with defined removed and you will see the different result.