use 5.012;
use warnings;
use Errno qw/EINTR EAGAIN/;

# ... initialization stuff ...

RECORD_LOOP: while (1) {
  my $record = "";
  READ_LOOP: while (length($record) < 794) {
    my $_;
    given ($nh->read_entity(my $chunk, 794 - length($record))) {
      last RECORD_LOOP when ($_ == 0 and length($record) == 0);
      next READ_LOOP when ($_ ~~ [ undef, -1 ] and $! ~~ [ EINTR, EAGAIN ]);

      die "I/O error: $!" when ([ undef, -1 ]);
      die "Unexpected end of file" when (0);

      default {
        $record .= $chunk;
      }
    }
  }

  # ... record processing ...
}