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in reply to Autovivification sucking the life out of me

Your autovivification happens here: 
see_args( @ENV{qw(TMPDIR TEMP TMP)} );

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If you explicitly tell perl to vivify $ENV{TMPDIR} it will do so. It's not the call doing it, but the parameters you pass in your call.
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

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Re^2: Autovivification sucking the life out of me
by Perl Mouse (Chaplain) on Oct 14, 2005 at 12:34 UTC
    Whenever a non-existing hash or array element is used in anything but a simple context, it's autovivificated. The *default* is to autovivificate - it's just that the exceptions happen to be the most often used cases, so it's a surprise when perl autovivifies. 
    #!/usr/bin/perl

use strict;
use warnings;

my %hash;

sub X {}
my $x;

$x = exists $hash{key1};
printf "key1 %sautovivified\n",  exists $hash{key1} ? "" : "not ";

$x = $hash{key2};
printf "key2 %sautovivified\n",  exists $hash{key2} ? "" : "not ";

$x = exists $hash{key4}{key3};
printf "key3 %sautovivified\n",  exists $hash{key4}{key3} ? "" : "not 
+";
printf "key4 %sautovivified\n",  exists $hash{key4} ? "" : "not ";

($x) = @hash{key5};
printf "key5 %sautovivified\n",  exists $hash{key5} ? "" : "not ";

($x) = @hash{"key6", "key7"};
printf "key6 %sautovivified\n",  exists $hash{key6} ? "" : "not ";
printf "key7 %sautovivified\n",  exists $hash{key7} ? "" : "not ";

X($hash{key8});
printf "key8 %sautovivified\n",  exists $hash{key8} ? "" : "not ";

X(@hash{key9});
printf "key9 %sautovivified\n",  exists $hash{key9} ? "" : "not ";

X(@hash{"keyA","keyB"});
printf "keyA %sautovivified\n",  exists $hash{keyA} ? "" : "not ";
printf "keyB %sautovivified\n",  exists $hash{keyB} ? "" : "not ";  
__END__
key1 not autovivified
key2 not autovivified
key3 not autovivified
key4 autovivified
key5 not autovivified
key6 not autovivified
key7 not autovivified
key8 not autovivified
key9 autovivified
keyA autovivified
keyB autovivified

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    Perl --((8:>*
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      Wow, thanks for the examples. It would seem "simple usage" is not what people would assume it is. I will go file a bug report against PathTools, but it may be instructive to see how easy it is to fall into this trap. Here is the line of code in File::Spec::Win32.pm that caused me all the grief: 
        $tmpdir = $_[0]->_tmpdir( @ENV{qw(TMPDIR TEMP TMP)},
                            'SYS:/temp',
                            'C:\system\temp',
                            'C:/temp',
                            '/tmp',
                            '/'  );

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      Sure looks innocent, huh? Of course the 'fix' will be to change the references to 'simple' ones, 
        $tmpdir = $_[0]->_tmpdir( $ENV{TMPDIR},
                            $ENV{TEMP},
                            $ENV{TMP},

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      And in answer to Skeeve, yes, I am a "bug-magnet"

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Re^2: Autovivification sucking the life out of me
by shenme (Priest) on Oct 14, 2005 at 07:51 UTC
    But I am not explicitly telling Perl to autovivify, rather I am (intending to) just reference the values, in this case using a slice.

    Refer to my simpler example above. Using see_args( @foo{qw(BAR BAZ)} ); causes result: 

    at start: BAR '<absent>'
at end: BAR '<undef>'

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    But changing that to see_args( $foo{BAR}, $foo{BAZ} ); results in: 
    at start: BAR '<absent>'
at end: BAR '<absent>'

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    This is most perplexing to me.
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      Tell you what: It's perplexing to me the other way around!

      Until 2 days ago I thought something like
      if ($foo{'BAR'}) ...
      would already vivify that hash entry.

      But then I learned in the Chatter Box that it doesn't in perl 5.8 (or maybe a bit earlier).

      And now I did a test. Compare the output 

      use Data::Dumper;
%foo=(BAZ=>1);
print Dumper \%foo if $foo{BAR} || 1;
print Dumper \%foo if @foo{BAR,BAZ} || 1;

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      Output:

      $VAR1 = {
                'BAZ' => 1
              };
      $VAR1 = {
                'BAZ' => 1
              }; 

      use Data::Dumper;
%foo=(BAZ=>1);
print Dumper \%foo if call($foo{BAR});
print Dumper \%foo if call(@foo{BAR,BAZ});
sub call { return 1 }

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      Output:

      $VAR1 = {
                'BAZ' => 1
              };
      $VAR1 = {
                'BAR' => undef,
                'BAZ' => 1
              };

      So it's really the fact that the slice is used in a subroutine call. When you replace call with print, autovivification does not happen

      So now I ask myself: Is there a bug and you are certified bugfinder!? ;-)

      s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
      +.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e
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