Works fine for me:

while (<>)
{
    print "got $_";
}
[download]

1   
got 1
2
got 2
0
got 0
5
got 5
[download]

Update: see perlop2:

In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see if it is defined. The defined test avoids problems where line has a string value that would be treated as false by perl e.g. ``'' or ``0'' with no trailing newline.