# Perl
Perl is a scripting [programming language](programming_language.md) created by Larry Wall in 1987. Originally meant
for text processing (as a replacement for [AWK](awk.md)) it latter became a general purpose language. It is quite
[bloated](bloat.md) and has only a single relevant implementation (written in C). Also it is known for its syntax,
which can be considered as "inelegant" and leading to "write-only" programs, also the extensive use of sigils;
marks used before identifiers used as a way of communicating the data type of the underlying identifiers; using
the wrong sigil can lead to unexpected type conversion.

Perl includes powerful [regex](regular_expression.md) capabilities, although its regex engine is susceptible to catastrophic
backtracking.

It is shipped by default on most [Linux](linux.md) distributions, so it could be used as a replacement for [shell scripts](shell_script.md)
in some applications, but writing such a program in a more efficient language such as [C](c.md) is preferred.

## Notes
### Stricter language and warnings
This two lines meant to be put at the top of a script serve as boilerplate to make Perl more strict and saner:
```perl
use warnings;
use strict;
```

### Sigils
- `$`: scalar value; numbers, strings...
- `@`: [array](array.md)
- `%`: [hash table](hash_table.md)
- `&`: functions; rare in "modern" Perl code.

### Feature toggling
For its credit, Perl has a decent level of backwards compatibility compared to other scripting
languages, so some features that may cause problems with older code are disabled by default. Perl
allows enabling a set of features manually, for example here we enable the `say` function:
```perl
use feature 'say';      # this can also be an array of features...
```
More conveniently, instead of enabling each feature manually one can simply pick a certain version
and all of its features will be enabled:
```perl
use v5.34;              # enables `say`, `defer`, `try`-`catch` and more...
```

### Automatically die on any error
This makes a Perl script commit suicide if any error is raised (akin to shell script `set -e`):
```perl
use autodie;
```

## Miscellaneous weirdness
- When calling the Perl interpreter directly, any [shebang](shebang.md) at the top of the script that doesn't actually call
the Perl interpreter itself gets executed, just like the kernel would interpret the shebang.
- ...

## Examples
### Hello world
```perl
# with the classical `print`:
print "Hello, World!\n";
# with `say`:
say "Hello, World!";
```

### Factorial
```perl
use warnings;
use strict;
use bignum;     # optional

sub fact {
	my $n = shift;
	my $res = 1;
	while ($n > 0) {
		$res *= $n;
		--$n;
	}
	return $res;
}

my @ns = (5, 0, 39, 1, 4, 10);
foreach my $n (@ns) {
	printf "fact(%d) = %s\n", $n, fact($n);
}
```

### As a replacement for `sed`
It is possible to use Perl as `sed` to benefit from its more powerful regex:
```sh
# you can also pass `-i` to do inline replacement, like in sed
perl -p -e 's/foo/bar/g' files...
```

TODO

## Resources
- [Perl official website](https://www.perl.org/)
- [Perl Maven](https://perlmaven.com/): page containing tutorials and more