return undef if not defined $in_file;
if (-e $in_file) {
  if (-r $in_file) {
    open F, "<", $in_file or die "Can't open $in_file: $!"
# etc
  } else {
    warn "can't read $in_file\n";
    return undef;
  }
} else {
  warn "bad file $in_file submitted, but does not exist\n";
}