Question

I have a program which accepts a perl data structure which is intended to be a Storable scalar. Is there a way to test if the scalar is a valid Storable object, without dying if it isn’t?

For instance, if I do:

use Storable qw(freeze thaw);
my $ref = thaw("lol_not_storable")

I get back “Storable binary image v54.111 more recent than I am (v2.8) at /usr/local/lib/perl/5.12.4/Storable.pm line 420, at test.pl line 5”

I would like to figure out if it is possible to cleanly handle these exceptions without eval. Is it possible without rewriting the Storable Perl Module?

Answer 1

eval { thaw("lol_not_storable"); };

is not the same thing as

eval qq/thaw("lol_not_storable");/;

in that Perl has ample chance to parse the first, but waits to parse the second. Observe, the below is a compile error:

use 5.014;
use strict;
use warnings;

say 'Would print without compile error';
eval { $i++; };
^D

Global symbol "$i" requires explicit package name at - line 8.
Execution of - aborted due to compilation errors.

Whereas eval '$i++' wouldn’t be. I think most of the discouragement you have heard about eval is more of the latter type and not of the former. The latter evaluates a string as code, the former mainly tells Perl “don’t die.”

Here’s the string version:

use 5.014;
use strict;
use warnings;

say 'Would print without compile error';
eval ' $i++;';

Outputs:

Would print without compile error

The code still fails to compile, but only when it’s eval-ed, and only has effect when I check $@, which reads:

$@= 'Global symbol "$i" requires explicit package name at (eval 24) line 1.
'