unless (defined( $line = <> )) {
      die $! if $!;      # $! is only current and valid in the aftermath of a failure
      last; # reached EOF
 }

##</code><code>##

$line = <>;
if ($!) {
  print "Ohh noo, $!";  # an indefinite false-positive
}

##</code><code>##

$ touch test;  # create the dummy

$ perl -Mstrict -Wle 'undef $!; open my $fh, "<", "test" or warn "warn : $!"; print "after : $!"'
after : Inappropriate ioctl for device

$ rm test;

$perl -Mstrict -Wle 'undef $!; open my $fh, "<", "test" or warn "warn : $!"; print "after : $!"
warn : No such file or directory at -e line 1.
after : No such file or directory