=head1 DESCRIPTION
A Perl script consists of a sequence of declarations and statements.
-The only things that need to be declared in Perl are report formats
-and subroutines. See the sections below for more information on those
-declarations. All uninitialized user-created objects are assumed to
-start with a C<null> or C<0> value until they are defined by some explicit
-operation such as assignment. (Though you can get warnings about the
-use of undefined values if you like.) The sequence of statements is
-executed just once, unlike in B<sed> and B<awk> scripts, where the
-sequence of statements is executed for each input line. While this means
-that you must explicitly loop over the lines of your input file (or
-files), it also means you have much more control over which files and
-which lines you look at. (Actually, I'm lying--it is possible to do an
-implicit loop with either the B<-n> or B<-p> switch. It's just not the
-mandatory default like it is in B<sed> and B<awk>.)
-
-=head2 Declarations
+The sequence of statements is executed just once, unlike in B<sed>
+and B<awk> scripts, where the sequence of statements is executed
+for each input line. While this means that you must explicitly
+loop over the lines of your input file (or files), it also means
+you have much more control over which files and which lines you look at.
+(Actually, I'm lying--it is possible to do an implicit loop with
+either the B<-n> or B<-p> switch. It's just not the mandatory
+default like it is in B<sed> and B<awk>.)
Perl is, for the most part, a free-form language. (The only exception
to this is format declarations, for obvious reasons.) Text from a
context, and C++ C<//> comments just look like a null regular
expression, so don't do that.
+=head2 Declarations
+
+The only things you need to declare in Perl are report formats
+and subroutines--and even undefined subroutines can be handled
+through AUTOLOAD. A variable holds the undefined value (C<undef>)
+until it has been assigned a defined value, which is anything
+other than C<undef>. When used as a number, C<undef> is treated
+as C<0>; when used as a string, it is treated the empty string,
+C<"">; and when used as a reference that isn't being assigned
+to, it is treated as an error. If you enable warnings, you'll
+be notified of an uninitialized value whenever you treat C<undef>
+as a string or a number. Well, usually. Boolean ("don't-care")
+contexts and operators such as C<++>, C<-->, C<+=>, C<-=>, and
+C<.=> are always exempt from such warnings.
+
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time. Typically all the declarations are put at
the beginning or the end of the script. However, if you're using
-lexically-scoped private variables created with C<my()>, you'll have to make sure
+lexically-scoped private variables created with C<my()>, you'll
+have to make sure
your format or subroutine definition is within the same block scope
as the my if you expect to be able to access those private variables.
refers to the innermost enclosing loop. This may include dynamically
looking back your call-stack at run time to find the LABEL. Such
desperate behavior triggers a warning if you use the B<-w> flag.
+Unlike a C<foreach> statement, a C<while> statement never implicitly
+localises any variables.
If there is a C<continue> BLOCK, it is always executed just before the
conditional is about to be evaluated again, just like the third part of a
https://perl5.git.perl.org / perl5.git / blobdiff