Add content to new concepts

concepts/booleans-and-conditionals/about.md

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+# About
+
+## Booleans
+
+Boolean values exist in Perl, however they are seldom used explicitly.
+They typically come as the result of a comparison operation, e.g, `2 > 1` or `'foo' eq 'bar'`, or negation, e.g., `!0`.
+
+`1` and `0` are commonly used to represent true and false in boolean contexts.
+With `use v5.40` or newer, the functions `true` and `false` are included to provide explicit boolean values.
+
+The following are considered false in Perl:
+
+```perl
+0;   # The number 0.
+'0'; # A single 0 in a string.
+'';  # An empty string.
+();  # An empty list.
+undef;
+```

concepts/booleans-and-conditionals/introduction.md

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+# Introduction
+
+## Booleans
+
+Boolean values exist in Perl, however they are seldom used explicitly.
+They typically come as the result of a comparison operation, e.g, `2 > 1` or `'foo' eq 'bar'`, or negation, e.g., `!0`.
+
+`1` and `0` are commonly used to represent true and false in boolean contexts.
+With `use v5.40` or newer, the functions `true` and `false` are included to provide explicit boolean values.
+
+The following are considered false in Perl:
+
+```perl
+0;   # The number 0.
+'0'; # A single 0 in a string.
+'';  # An empty string.
+();  # An empty list.
+undef;
+```

concepts/object-orientation/about.md

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+# About
+
+## Classes
+
+Class-based object orientation is a paradigm often used in Perl.
+Traditionally, classes in Perl are created by using `bless` to associate a reference with a package.
+
+```perl
+package Foo;
+
+sub new ($class, %args) {
+    return bless {%args{bar}}, $class;
+}
+
+sub bar ($self) {
+    return $self->{bar};
+}
+```
+
+Perl `v5.38` introduced the `class` keyword which brings additional OOP features to core Perl.
+
+```perl
+use feature qw<class>;
+
+class Foo;
+
+field $bar :param;
+
+method bar () {
+    return $bar;
+}
+```
+
+Additionally, Perl has numerous modules available on CPAN implementing OOP features.
+Among the most well known are `Moo` and `Moose`.
+
+```perl
+package Foo;
+
+use Moo;
+
+has bar => (is => 'ro');
+```
+
+The arrow operator (`->`) is used to call methods on class names and objects.
+All of the above can be used as follows:
+
+```perl
+# Object creation
+my $object = Foo->new(bar => 1);
+
+# Method call
+say $object->bar;
+```

concepts/object-orientation/introduction.md

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+# Introduction
+
+## Classes
+
+Class-based object orientation is a paradigm often used in Perl.
+Traditionally, classes in Perl are created by using `bless` to associate a reference with a package.
+
+```perl
+package Foo;
+
+sub new ($class, %args) {
+    return bless {%args{bar}}, $class;
+}
+
+sub bar ($self) {
+    return $self->{bar};
+}
+```
+
+Perl `v5.38` introduced the `class` keyword which brings additional OOP features to core Perl.
+
+```perl
+use feature qw<class>;
+
+class Foo;
+
+field $bar :param;
+
+method bar () {
+    return $bar;
+}
+```
+
+Additionally, Perl has numerous modules available on CPAN implementing OOP features.
+Among the most well known are `Moo` and `Moose`.
+
+```perl
+package Foo;
+
+use Moo;
+
+has bar => (is => 'ro');
+```
+
+The arrow operator (`->`) is used to call methods on class names and objects.
+All of the above can be used as follows:
+
+```perl
+# Object creation
+my $object = Foo->new(bar => 1);
+
+# Method call
+say $object->bar;
+```