I was referring to being wary when $line=<FH> is used on its own. To illustrate:

$ perl -e 'print "with_ending\nno_ending"' > a.txt
$ perl -MData::Dumper
    open(FH, "<", "a.txt") or die $!;
    my $line1 = <FH>;
    my $line2 = <FH>;
    my $line3 = <FH>;
    close FH;

    $Data::Dumper::Useqq = 1;
    print Dumper [$line1, $line2, $line3];
__END__
$VAR1 = [
          "with_ending\n",
          "no_end",
          undef
        ];
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We see that there is a possibility that a line read ($line3 in this case) may return undef. That's what I meant: we need to be wary that a line read will not always return defined.