I'm reading Beginning Perl by Simon Cozens and in Chapter 8 - Subroutines he states that "subroutines" are user functions, while print, open, split, etc. are built-in operators or functions.
What are they? Are they really built-in, language-intrinsic features (like C's sizeof operator) or are they, actually, subroutines/functions of the main module?
If they're subroutines, are while, for, unless, etc. also subroutines? What about the operators like +, -, eq, etc.?
print, open, split are not subroutines. They do not result in sub calls. They are not even present in the symbol table (in main:: or otherwise, although you can refer to them as CORE::split, etc), and one cannot get a reference to their code (although work is being done to create proxy subs for them in CORE:: for when you want to treat them as subroutines). They are operators just like +.
$ perl -MO=Concise,-exec -e'sub f {} f()'
1 <0> enter
2 <;> nextstate(main 2 -e:1) v:{
3 <0> pushmark s
4 <#> gv[*f] s
5 <1> entersub[t3] vKS/TARG,1 <--- sub call
6 <@> leave[1 ref] vKP/REFC
-e syntax OK
$ perl -MO=Concise,-exec -e'split /;/'
1 <0> enter
2 <;> nextstate(main 1 -e:1) v:{
3 </> pushre(/";"/) s/64
4 <#> gvsv[*_] s
5 <$> const[IV 0] s
6 <@> split[t2] vK <--- not a sub call
7 <@> leave[1 ref] vKP/REFC
-e syntax OK
$ perl -MO=Concise,-exec -e'$x + $y'
1 <0> enter
2 <;> nextstate(main 1 -e:1) v:{
3 <#> gvsv[*x] s
4 <#> gvsv[*y] s
5 <2> add[t3] vK/2 <--- Just like this
6 <@> leave[1 ref] vKP/REFC
-e syntax OK
They are known by a variety of names:
And most are considered to be one of the following:
Subroutines are often called functions (as they are in C and C++), so "function" is an ambiguous word. This ambiguity appears to be the basis of your question.
As for while, for, unless, etc, they are keywords used by flow control statements
while (f()) { g() }
and statement modifiers
g() while f();
The Perl keywords are those defined in the regen/keywords.pl file within the Perl source distribution. These are:
__FILE__, __LINE__, __PACKAGE__, __DATA__, __END__, AUTOLOAD, BEGIN, UNITCHECK, CORE, DESTROY, END, INIT, CHECK, abs, accept, alarm, and, atan2, bind, binmode, bless, break, caller, chdir, chmod, chomp, chop, chown, chr, chroot, close, closedir, cmp, connect, continue, cos, crypt, dbmclose, dbmopen, default, defined, delete, die, do, dump, each, else, elsif, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, eof, eq, eval, exec, exists, exit, exp, fcntl, fileno, flock, for, foreach, fork, format, formline, ge, getc, getgrent, getgrgid, getgrnam, gethostbyaddr, gethostbyname, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getpeername, getpgrp, getppid, getpriority, getprotobyname, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyname, getservbyport, getservent, getsockname, getsockopt, given, glob, gmtime, goto, grep, gt, hex, if, index, int, ioctl, join, keys, kill, last, lc, lcfirst, le, length, link, listen, local, localtime, lock, log, lstat, lt, m, map, mkdir, msgctl, msgget, msgrcv, msgsnd, my, ne, next, no, not, oct, open, opendir, or, ord, our, pack, package, pipe, pop, pos, print, printf, prototype, push, q, qq, qr, quotemeta, qw, qx, rand, read, readdir, readline, readlink, readpipe, recv, redo, ref, rename, require, reset, return, reverse, rewinddir, rindex, rmdir, s, say, scalar, seek, seekdir, select, semctl, semget, semop, send, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shift, shmctl, shmget, shmread, shmwrite, shutdown, sin, sleep, socket, socketpair, sort, splice, split, sprintf, sqrt, srand, stat, state, study, sub, substr, symlink, syscall, sysopen, sysread, sysseek, system, syswrite, tell, telldir, tie, tied, time, times, tr, truncate, uc, ucfirst, umask, undef, unless, unlink, unpack, unshift, untie, until, use, utime, values, vec, wait, waitpid, wantarray, warn, when, while, write, x, xor, y.
The perlsyn, perlop, and perlsub manpages are required reading, followed perhaps by the perlfunc manpage. To learn how to override builtin operators used with objects, see the overload manpage.
if and unless. Perl has very few of those absolutely inviolate keywords. Most of Perl’s keywords from regen/keywords.pl can be indeed be overridden through one (or more) of three ways: by importing a function of that name from another package (as in chdir or open) or else through either tying (as in keys or pop) or operator overloading (as in print, cmp, or qq). Which is which requires experience with the language to understand.The built-in operators are not Perl subroutines. For example,
#!/usr/bin/perl
use strict;
use warnings;
sub Foo { print "In foo\n"; }
my $ref;
$ref = \&Foo;
$ref->();
$ref = \&print;
$ref->();
The first $ref->(); is an indirect call; it prints "In foo".
The second one produces a warning:
Undefined subroutine &main::print called at ./tmp.pl line 14
because print is not the name of a subroutine.
\&CORE::builtin is being added in 5.16. It'll autovivify a sub that calls the builtin.Simply think of "built-in functions" as functions that you did not create. Easy right? :-)
&. Except you can't goto them. (and I'm sure I'm missing some)
perldoc perlfuncandperldoc -f <function name>.