strict, warnings and diagnostics

use strict

Perl is permissive by default: an undeclared variable simply springs into existence, and a typo in a variable name silently creates a second variable. use strict turns those mistakes into hard errors.

use strict; use warnings; my $name = "Ada"; my @items = ("pen", "cup"); my %count = (pen => 1); print $name, "\n"; print scalar @items, "\n"; print $count{pen}, "\n";

Every variable is introduced with my, which gives it lexical scope that ends at the closing brace of the enclosing block. Drop the declaration and the program stops before it does any damage:

use strict; $name = "Ada";

Global symbol "$name" requires explicit package name (did you forget to declare "my $name"?) at demo.pl line 2. Execution of demo.pl aborted due to compilation errors.

strict is really three switches in one: vars requires declarations, subs requires parentheses or a predeclared name for every function call, and refs forbids symbolic references. The default use strict enables all three, and that is what you want.

use warnings

Warnings are the other half: they do not stop the program, they tell you that a value was missing, a string looked like a number, or a result was thrown away. Most of them point at real bugs.

use warnings; my $total; print $total + 1, "\n"; my @scores = (10, 20); print $scores[5], "\n";

Use of uninitialized value $total in addition (+) at demo.pl line 4. Use of uninitialized value within @scores in print at demo.pl line 7.

Instead of the old -w flag on the shebang line, put use warnings in the file. The flag applies to every module the program loads, including ones you did not write; the pragma applies only to the file that uses it.

In development you can make selected warnings fatal, so the program dies at the first sign of trouble instead of printing a line and carrying on:

use warnings FATAL => 'all'; my $x; print $x * 2;

Use of uninitialized value $x in multiplication (*) at demo.pl line 4.

Turn fatal warnings off before shipping a tool that reads untrusted data, or one odd input will end the run.

diagnostics and splain

Some Perl messages are famously terse. The diagnostics pragma replaces each one with a paragraph that explains the cause and the usual fix:

use diagnostics; my %opts; print $opts{port} + 1, "\n";

Use of uninitialized value $opts{"port"} in addition (+) at demo.pl line 4 (#1) (W uninitialized) An undefined value was used as if it were already defined. It was interpreted as a "" or a 0, but maybe it was a mistake. To suppress this warning assign a defined value to your variables.

Loading diagnostics permanently slows a program down, so keep it for a debugging session and delete it afterwards. For one run you can ask the interpreter instead:

perl -Mdiagnostics script.pl

The same text lives in perldoc perldiag, organised by message. Search that page when a message makes no sense: it lists the severity letter, the cause, and the cure for each one.

Syntax checks and perlcritic

You do not have to run a program to know whether it compiles. The -c flag compiles the file and stops, which catches typos, missing modules and broken braces in a second:

perl -c script.pl

script.pl syntax OK

Two more switches help when you cannot see why a line behaves the way it does. -MO=Deparse prints the program after Perl has parsed it, which shows how precedence really grouped your expression. -MData::Dumper can dump a variable from a one-liner during a debug session.

perl -MO=Deparse script.pl

For style and risky constructs, install Perl::Critic and run it at a gentle severity. It flags things like two-argument open, bareword filehandles, and string eval, each with the policy name that explains the rule.

perlcritic --severity 3 script.pl

Code before strictures are enabled at line 3, column 1. See page 429 of PBP. (Severity: 5)

Not every policy is worth obeying in every file. Choose the severity you can live with, read the policy documentation before you change code to please the tool, and leave a comment when you deliberately break a rule.

Next: Options with Getopt::Long

Article author: Arthur Isaev