- open (my $F, $file) or die "cannot open file $!";
+ open (my $F), '<', $file or die "cannot open file $!";

##</code><code>##

open (my $F), '<', $file
  or warn "cannot open file '$file': $!";
defined $F
  or next;

##</code><code>##

open (my $F), '<', $file
  or warn "cannot open file '$file': $!" and next;