XooR has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks, I constructed this simple program that will show what is the problem. 
#!/usr/bin/perl

use strict;
use warnings;

use Module::CoreList;
use Data::Dumper;

print Dumper( $] );
 # Prints "5.010000" on my version of perl
print Dumper( $Module::CoreList::version{ $] } );
 # Prints undef
print Dumper( $Module::CoreList::version{ "$]" } );
 # Prints undef
print Dumper( $Module::CoreList::version{ 5.010000 } ); 
 # Prints "correct" list ...

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I don't understand why accessing hash with $] is different from accessing with it's value (in my case 5.010000)?

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Re: Problem with accessing hash Module::CoreList::version with $]
by betterworld (Curate) on Apr 24, 2009 at 09:57 UTC

    Apparently, this special variable is only a string in perl 5.10, even though the documentation seems to suggest that it be a floating point number. If you use $]+0 in your example, it'll work (which is kind of strange because ordinary hash keys are converted to strings anyway, but in this case I think we are dealing with a tied hash).

    % perl5.8.8 -we 'use Devel::Peek; Dump $]' 
SV = PVNV(0x9f664d0) at 0x9f63ed0            
  REFCNT = 1
  FLAGS = (NOK,POK,READONLY,pNOK,pPOK)
  IV = 0
  NV = 5.008008                       #  <-- in 5.8 it was a number
  PV = 0x9f7d250 "5.008008"\0
  CUR = 8
  LEN = 12

% perl5.10.0 -we 'use Devel::Peek; Dump $]'  
SV = PV(0x9457198) at 0x946a610
  REFCNT = 1
  FLAGS = (POK,READONLY,pPOK)
  PV = 0x945e538 "5.010000"\0
  CUR = 8
  LEN = 12

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      betterworld sad:
      Apparently, this special variable is only a string in perl 5.10, even though the documentation seems to suggest that it be a floating point number. If you use $]+0 in your example, it'll work (which is kind of strange because ordinary hash keys are converted to strings anyway, but in this case I think we are dealing with a tied hash).
      Thanx, that's solved it. I will read about tied hashes and see if this is a problem.
[reply]
      Actually that makes no difference, works just fine in 5.10. His problem is he expect Data::Dumper output to be identical. Maybe if he use $Data::Dumper::Sortkeys ... or this 
      #!/usr/bin/perl

use strict;
use warnings;

use Module::CoreList;
use Data::Dumper;

$Data::Dumper::Sortkeys=1;
print Dumper(
    $Module::CoreList::version{ $] },

    $Module::CoreList::version{ "$]" },
    $Module::CoreList::version{ 5.010000 }
);
__END__
$VAR1 = {
          'AnyDBM_File' => '1.00',
.....
        };
$VAR2 = $VAR1;
$VAR3 = $VAR1;

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Re: Problem with accessing hash Module::CoreList::version with $]
by Anonymous Monk on Apr 24, 2009 at 10:02 UTC
    How are you comparing? Visually? That is a bad idea ;) 
    #!/usr/bin/perl

use strict;
use warnings;

use Module::CoreList;


print qq{
$Module::CoreList::version{ $] } 
$Module::CoreList::version{ "$]" }
$Module::CoreList::version{ 5.010000 }

};
__END__
HASH(0x1a97888)
HASH(0x1a97888)
HASH(0x1a97888)

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    See how they all reference the same hash? They're all the same 
    pmvers Module::CoreList
2.17

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      If I run your script with 5.8.8, I get the output

      
HASH(0x96574a4) 
HASH(0x96574a4)
HASH(0x9628550)

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      For 5.10.0, I get

      Use of uninitialized value within %Module::CoreList::version in concat
+enation (.) or string at test.pl line 7.
Use of uninitialized value in concatenation (.) or string at test.pl l
+ine 7.

 

HASH(0x84c82b8)

      [download]

      Module::CoreList is version 2.15.

      Update: For the 5.10.0 case, Module::CoreList's version is 2.13.

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