in reply to Re: How could I check if a variable is 'read-only'?
in thread How could I check if a variable is 'read-only'?

Cleaner code is: 
sub is_readonly {
    eval {my $save=$_[0]; $_[0]=$save;};
    return $@ =~/read.?only/;
}

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Scalar::Util did not work for me.

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Re^3: How could I check if a variable is 'read-only'?
by syphilis (Archbishop) on Jan 02, 2019 at 05:02 UTC
    Scalar::Util did not work for me

    I would expect Scalar::Util to perform the task flawlessly.
    Could you provide an example script that demonstrates its failing ?

    Cheers,
    Rob
[reply]

      There does look like one possibility. Perlguts mentions that with Perl 5.16 and earlier: Copy-on-write (see the next section) shared a flag bit with read-only scalars. So the only way to test whether sv_setsv , etc., will raise a "Modification of a read-only value" error in those versions is: SvREADONLY(sv) && !SvIsCOW(sv)

      Scalar::Util doesn't do anything special for 5.16 or earlier, so it could have false positives in that case. Of course, 5.18 (where it's fixed) was released in 2013 :)

[reply]

      What do you make of this?

      #!/usr/bin/env perl

use strict;
use warnings;
use JSON;
use Scalar::Util qw/ readonly /;

my $data = decode_json('{ "val":false }');
print "readonly: ", readonly($data->{val}) ? 'yes' : 'no', "\n";
bless $data->{val};

      [download]

      That produces this for me (on Perl 5.18.2):

      $./try
readonly: no
Modification of a read-only value attempted at ./try line 10.

      [download]
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[d/l]
[select]
        cant reproduce that here, try upgrading perl and or json module
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