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  sub assert_valid_isbn {
    local($_) = @_;
...
    @g == 4 or die "'$_[0]' is broken into the wrong number of groups"
+;
    join('',@g) == 10 or die "'$_[0]' contains the wrong number of dig
+its.";
  }

  for my $isbn ( @isbns ) {
    eval {
...
    };
    $@ and warn $@;
  };