santhosh_89 has asked for the wisdom of the Perl Monks concerning the following question:

I want to make array/hash references from scalar value. I used eval method to convert to perl structure. I Used character inside the scalar value .it will not be single quote in value field as
( $a="a,b,{perl=>monk}").So the eval method returns null value.If i use single quote in all the values.Eval will return the correct value.Why the single quote is needed inside the scalar value (my $a="'a','b',{'perl'=>'monk'}" ),already The value is with in double quotes. otherwise anyone knows any cpan modules to solve this problem.The problems is if the string found [,{,I need to change that value to hash({) or array([) references. 
     The eval is given null value.
  my $a="[a,b,{perl=>monk}]";
  my $b=eval $a;
  print Dumper($b);
      
     Following coding is working fine.
  my $a="['a','b',{'perl'=>'monk'}]";
  my $b=eval $a;
  print Dumper($b);

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Replies are listed 'Best First'.
Re: How to make array references from scalar value
by ELISHEVA (Prior) on Jul 29, 2009 at 11:25 UTC

    You need the single quotes inside the double quotes because the stuff inside the double quotes must be valid Perl code. If your module or script has use strict, [a,b,{perl=>monk}] is not a valid expression. But ['a','b',{perl=>'monk'}] and ['a','b',{'perl'=>'monk'}] are.

    Best, beth

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Re: How to make array references from scalar value
by moritz (Cardinal) on Jul 29, 2009 at 11:28 UTC
    If you print $a in first version, it says a, b, {perl => monk }. If you eval it, it is interpreted as a perl program.

    In a perl program with strict in effect, a, b and monk are forbidden. The eval doesn't care how you constructed the string, it just works with what it sees.

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Re: How to make array references from scalar value
by Anonymous Monk on Jul 29, 2009 at 11:26 UTC
    null? you mean undef. You're supposed to show your real code, because the code you've shown works as you expect 
    #!/usr/bin/perl --


use Data::Dumper;
{
    my $a="[a,b,{perl=>monk}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

{
    my $a="['a','b',{'perl'=>'monk'}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

{
    use strict;
    use warnings;

    my $a="[a,b,{perl=>monk}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

__END__
$VAR1 = [
          'a',
          'b',
          {
            'perl' => 'monk'
          }
        ];
$VAR1 = [
          'a',
          'b',
          {
            'perl' => 'monk'
          }
        ];
Bareword "a" not allowed while "strict subs" in use at (eval 3) line 1
+.
Bareword "b" not allowed while "strict subs" in use at (eval 3) line 1
+.
Bareword "monk" not allowed while "strict subs" in use at (eval 3) lin
+e 1.
$VAR1 = undef;

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    It only "breaks" under strict subs (my last example)
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Re: How to make array references from scalar value
by Anonymous Monk on Jul 29, 2009 at 11:39 UTC 
    #!/usr/bin/perl   --

use strict;
use warnings;

use Safe;


my $result = Safe->new->reval("[a,b,{perl=>monk}]");

use Data::Dumper;
print Dumper($result);

__END__
$VAR1 = [
          'a',
          'b',
          {
            'perl' => 'monk'
          }
        ];

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    Safe Undumping
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      I checked the Safe modules from cpan.It has some bugs.It will cause the memory problem and it doesn't work under 5.5.5 version with version.pm.

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      Is it secure method to use this package and reval method in real time project.
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