And of course, it becomes essential when reading in a file, which could contain blank lines

Actually, no, Perl DWIMs this.

$ perl -MO=Deparse,-x7 -e'while($line=<$fh>){}'
while (defined($line = <$fh>)) {
    ();
}
-e syntax OK

But defined doesn’t help you in this case anyway, since undef is a legal value for an array element.

Makeshifts last the longest.