@characters = (
      [ "h" ],
      [ "e" ],
      [ "l", "p", "i" ],
      [ "l" ],
      [ "o" ],
);
sub spell{
    my $prefix = shift;
    my @c = @{+shift};
    unless( @c ){
        return print "$prefix\n";
    }
    foreach( @c ){
         spell("$prefix$_",@_);
    }
}
spell("",@characters);