I don't see your pure-Perl "Modification of a read-only value..." result on ActiveState 5.8.9 and Strawberry 5.14.4.1 (Update: likewise Strawberries 5.10.1.5 and 5.12.3.0), but there's something else I don't understand: 

c:\@Work\Perl\monks>perl -wMstrict -MData::Dumper -le
"sub modify { $_ = 5 for @_; }
 ;;
 my @a;
 @a = (undef) x 5;
 ;;
 modify(@a);
 print Dumper \@a;
 ;;
 my @b;
 $#b = 4;
 ;;
 modify(@b);
 print Dumper \@b;
 ;;
 my @c;
 $#c = 4;
 $c[ $#c ] = undef;
 ;;
 modify(@c);
 print Dumper \@c;
"
$VAR1 = [
          5,
          5,
          5,
          5,
          5
        ];

$VAR1 = [
          undef,
          undef,
          undef,
          undef,
          undef
        ];

$VAR1 = [
          undef,
          undef,
          undef,
          undef,
          5
        ];

[download]
Arrays @b and @c are still unwriteable (or at least unwritten, at least in part), but in a different way.   ?!?

Update: Taking the  @_ argument list out of the equation, I get expected results (thank Larry!) for the problematic cases above (for all Perl versions listed in the first paragraph): 

c:\@Work\Perl\monks>perl -wMstrict -MData::Dumper -le
"my @b;
 $#b = 4;
 ;;
 $_ = 5 for @b;
 print Dumper \@b;
 ;;
 my @c;
 $#c = 4;
 $c[ $#c ] = undef;
 ;;
 $_ = 6 for @c;
 print Dumper \@c;
"
$VAR1 = [
          5,
          5,
          5,
          5,
          5
        ];

$VAR1 = [
          6,
          6,
          6,
          6,
          6
        ];

[download]

Give a man a fish:  <%-{-{-{-<

In reply to Re^5: What does my @x= (undef)x7; do? by AnomalousMonk
in thread What does my @x= (undef)x7; do? by gepebril69

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