I think I'd recommend a "destructive test" on a copy of the string. Perhaps something like this:

  sub assert_valid_isbn {
    local($_) = @_;
    s/^ISBN\s*// or die "'$_' does not begin with 'ISBN'";
    y/- 0-9//cd and die "'$_[0]' contains invalid characters";
    /-/ && / / and die "'$_[0]' contains both hyphens and spaces";
    y/-/ /; # convert hyphens to spaces.
    my @g = split;
    @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.";
  }
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This doesn't return anything, but throws an exception for invalid values. You could use it like this:

  for my $isbn ( @isbns ) {
    eval {
      assert_valid_isbn( $isbn );
      process_this_isbn( $isbn );
    };
    $@ and warn $@;
  };
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jdporter
...porque es dificil estar guapo y blanco.