Three-argument form of open used and it is not available until perl 5.6.
If your code violates it you'll get a message:
Three-argument form of open used at ... Three-argument open is not available until perl 5.6.
but I think this is mostly a distraction.
Normally you should use use 3-argument open, but that does not work on perl before version 5.6 came out in 2000. If you have an environment where you need to use an older version of Perl, I'd suggest you run away. I am not sure what an older version of perl would do if it encountered a 3-argument open. Would in give a syntax error or would in misunderstand it? Probably the former so you don't really need a Perl::Critic policy for it.
Nevertheless this policy exists, and if you are not careful, and someone runs perl critic on your code with this module being installed you'll get a report. Just as I did.
Let's see an example that will trigger this report:
open my $fh, '<', 'file.txt'; open my FH, '<', 'file.txt';
If I run
$ perlcritic --single-policy Compatibility::ProhibitThreeArgumentOpen three_argument_open.pl
Three-argument form of open used at line 2, column 1. Three-argument open is not available until perl 5.6. (Severity: 5) Three-argument form of open used at line 4, column 1. Three-argument open is not available until perl 5.6. (Severity: 5)
Using old-style open is out of the question, so how could we make sure we don't get this report?
There are a number of ways to solve the report problem in the generic case, but in the specific case we can actually do better.
We can declare in the code that we require a version of perl which is version 5.6 or later.
If we add
or if we require some later version of Perl:
then we won't get the above policy violation report.
Having the minimum version number declared early in the code is actually a good idea. It will help you avoid needing to deal with reports about syntax errors when someone runs your code on a version of perl older than what you were expecting.
Setting minimum version
The requirement of a minimum version of perl for 3-argument open is just one special case among many cases when certain constructs require a minimum version of perl.
There is another Perl::Critic policy that will check the code for any such violation, and will report what is the minimum version of perl needed for a specific piece of code.
If we run
$ perlcritic --single-policy Compatibility::PerlMinimumVersionAndWhy three_argument_open.pl
We'll get the following report:
_Pulp__open_my_filehandle requires 5.006 at line 2, column 1. (no explanation). (Severity: 2) _three_argument_open requires 5.006 at line 2, column 1. (no explanation). (Severity: 2)
It explains which construct in your code set which minimum requirements.
will silence this policy as well.
I don't recommend the use of the Compatibility::ProhibitThreeArgumentOpen policy, but I recommend the Compatibility::PerlMinimumVersionAndWhy and the inclusion of minimum version numbers in your source code.