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

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

$ touch test;  # create the dummy

...
$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