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I'm sorry I'm not good at English. :-)

...
Before line 13, line 12 prints the intervening elements, treating
undef as null string. Then why does line 13 make such error? Is it a
bug? or...?

picard:~ [13:47:28]$ perl -le '$x[5]=5; $_*=10 for @x; print "@x"'
0 0 0 0 0 50
...
Modification of a read-only value attempted at -e line 1.
picard:~ [13:47:36]$ perl -le '@x=0..5; $_*=10 for grep 1, @x; print "
+@x"'
0 10 20 30 40 50

   my @array;
   $array[0] = 0;
   $array[9] = 9;

    SV = NULL(0x0) at 0x182d528
      REFCNT = 1
      FLAGS = ()

    SV = PVLV(0x18b3164) at 0x182d510
      REFCNT = 2
...
        Elt No. 1
        Elt No. 2
        Elt No. 3

    SV = NULL(0x0) at 0x224b30
      REFCNT = 2147479514
      FLAGS = (READONLY)

picard:~ [13:53:28]$ cat inc.pl
#!/usr/bin/perl -l
...
picard:~ [13:53:31]$ ./inc.pl
Use of uninitialized value in join or string at ./inc.pl line 11.
     667

picard:~ [14:15:22]$ cat inc.pl
#!/usr/bin/perl
...
SV = NULL(0x0) at 0x819ab20
  REFCNT = 1
  FLAGS = ()