perl.man

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The shell executes the second line as a normal shell command, and thus
starts up the
.I perl
interpreter.
On some systems $0 doesn't always contain the full pathname,
so the
.B \-S
tells
.I perl
to search for the script if necessary.
After
.I perl
locates the script, it parses the lines and ignores them because
the variable $running_under_some_shell is never true.
A better construct than $* would be ${1+"$@"}, which handles embedded spaces
and such in the filenames, but doesn't work if the script is being interpreted
by csh.
In order to start up sh rather than csh, some systems may have to replace the
#! line with a line containing just
a colon, which will be politely ignored by perl.
Other systems can't control that, and need a totally devious construct that
will work under any of csh, sh or perl, such as the following:
.nf

.ne 3
	eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
	& eval 'exec /usr/bin/perl -S $0 $argv:q'
		if 0;

.fi
.TP 5
.B \-u
causes
.I perl
to dump core after compiling your script.
You can then take this core dump and turn it into an executable file
by using the undump program (not supplied).
This speeds startup at the expense of some disk space (which you can
minimize by stripping the executable).
(Still, a "hello world" executable comes out to about 200K on my machine.)
If you are going to run your executable as a set-id program then you
should probably compile it using taintperl rather than normal perl.
If you want to execute a portion of your script before dumping, use the
dump operator instead.
Note: availability of undump is platform specific and may not be available
for a specific port of perl.
.TP 5
.B \-U
allows
.I perl
to do unsafe operations.
Currently the only \*(L"unsafe\*(R" operations are the unlinking of directories while
running as superuser, and running setuid programs with fatal taint checks
turned into warnings.
.TP 5
.B \-v
prints the version and patchlevel of your
.I perl
executable.
.TP 5
.B \-w
prints warnings about identifiers that are mentioned only once, and scalar
variables that are used before being set.
Also warns about redefined subroutines, and references to undefined
filehandles or filehandles opened readonly that you are attempting to
write on.
Also warns you if you use == on values that don't look like numbers, and if
your subroutines recurse more than 100 deep.
.TP 5
.BI \-x directory
tells
.I perl
that the script is embedded in a message.
Leading garbage will be discarded until the first line that starts
with #! and contains the string "perl".
Any meaningful switches on that line will be applied (but only one
group of switches, as with normal #! processing).
If a directory name is specified, Perl will switch to that directory
before running the script.
The
.B \-x
switch only controls the the disposal of leading garbage.
The script must be terminated with _\|_END_\|_ if there is trailing garbage
to be ignored (the script can process any or all of the trailing garbage
via the DATA filehandle if desired).
.Sh "Data Types and Objects"
.PP
.I Perl
has three data types: scalars, arrays of scalars, and
associative arrays of scalars.
Normal arrays are indexed by number, and associative arrays by string.
.PP
The interpretation of operations and values in perl sometimes
depends on the requirements
of the context around the operation or value.
There are three major contexts: string, numeric and array.
Certain operations return array values
in contexts wanting an array, and scalar values otherwise.
(If this is true of an operation it will be mentioned in the documentation
for that operation.)
Operations which return scalars don't care whether the context is looking
for a string or a number, but
scalar variables and values are interpreted as strings or numbers
as appropriate to the context.
A scalar is interpreted as TRUE in the boolean sense if it is not the null
string or 0.
Booleans returned by operators are 1 for true and 0 or \'\' (the null
string) for false.
.PP
There are actually two varieties of null string: defined and undefined.
Undefined null strings are returned when there is no real value for something,
such as when there was an error, or at end of file, or when you refer
to an uninitialized variable or element of an array.
An undefined null string may become defined the first time you access it, but
prior to that you can use the defined() operator to determine whether the
value is defined or not.
.PP
References to scalar variables always begin with \*(L'$\*(R', even when referring
to a scalar that is part of an array.
Thus:
.nf

.ne 3
    $days	\h'|2i'# a simple scalar variable
    $days[28]	\h'|2i'# 29th element of array @days
    $days{\'Feb\'}\h'|2i'# one value from an associative array
    $#days	\h'|2i'# last index of array @days

but entire arrays or array slices are denoted by \*(L'@\*(R':

    @days	\h'|2i'# ($days[0], $days[1],\|.\|.\|. $days[n])
    @days[3,4,5]\h'|2i'# same as @days[3.\|.5]
    @days{'a','c'}\h'|2i'# same as ($days{'a'},$days{'c'})

and entire associative arrays are denoted by \*(L'%\*(R':

    %days	\h'|2i'# (key1, val1, key2, val2 .\|.\|.)
.fi
.PP
Any of these eight constructs may serve as an lvalue,
that is, may be assigned to.
(It also turns out that an assignment is itself an lvalue in
certain contexts\*(--see examples under s, tr and chop.)
Assignment to a scalar evaluates the righthand side in a scalar context,
while assignment to an array or array slice evaluates the righthand side
in an array context.
.PP
You may find the length of array @days by evaluating
\*(L"$#days\*(R", as in
.IR csh .
(Actually, it's not the length of the array, it's the subscript of the last element, since there is (ordinarily) a 0th element.)
Assigning to $#days changes the length of the array.
Shortening an array by this method does not actually destroy any values.
Lengthening an array that was previously shortened recovers the values that
were in those elements.
You can also gain some measure of efficiency by preextending an array that
is going to get big.
(You can also extend an array by assigning to an element that is off the
end of the array.
This differs from assigning to $#whatever in that intervening values
are set to null rather than recovered.)
You can truncate an array down to nothing by assigning the null list () to
it.
The following are exactly equivalent
.nf

	@whatever = ();
	$#whatever = $[ \- 1;

.fi
.PP
If you evaluate an array in a scalar context, it returns the length of
the array.
The following is always true:
.nf

	scalar(@whatever) == $#whatever \- $[ + 1;

.fi
If you evaluate an associative array in a scalar context, it returns
a value which is true if and only if the array contains any elements.
(If there are any elements, the value returned is a string consisting
of the number of used buckets and the number of allocated buckets, separated
by a slash.)
.PP
Multi-dimensional arrays are not directly supported, but see the discussion
of the $; variable later for a means of emulating multiple subscripts with
an associative array.
You could also write a subroutine to turn multiple subscripts into a single
subscript.
.PP
Every data type has its own namespace.
You can, without fear of conflict, use the same name for a scalar variable,
an array, an associative array, a filehandle, a subroutine name, and/or
a label.
Since variable and array references always start with \*(L'$\*(R', \*(L'@\*(R',
or \*(L'%\*(R', the \*(L"reserved\*(R" words aren't in fact reserved
with respect to variable names.
(They ARE reserved with respect to labels and filehandles, however, which
don't have an initial special character.
Hint: you could say open(LOG,\'logfile\') rather than open(log,\'logfile\').
Using uppercase filehandles also improves readability and protects you
from conflict with future reserved words.)
Case IS significant\*(--\*(L"FOO\*(R", \*(L"Foo\*(R" and \*(L"foo\*(R" are all
different names.
Names which start with a letter may also contain digits and underscores.
Names which do not start with a letter are limited to one character,
e.g. \*(L"$%\*(R" or \*(L"$$\*(R".
(Most of the one character names have a predefined significance to
.IR perl .
More later.)
.PP
Numeric literals are specified in any of the usual floating point or
integer formats:
.nf

.ne 6
    12345
    12345.67
    .23E-10
    0xffff	# hex
    0377	# octal
    4_294_967_296

.fi
String literals are delimited by either single or double quotes.
They work much like shell quotes:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for \e\' and \e\e).
The usual backslash rules apply for making characters such as newline, tab,
etc., as well as some more exotic forms:
.nf

	\et		tab
	\en		newline
	\er		return
	\ef		form feed
	\eb		backspace
	\ea		alarm (bell)
	\ee		escape
	\e033		octal char
	\ex1b		hex char
	\ec[		control char
	\el		lowercase next char
	\eu		uppercase next char
	\eL		lowercase till \eE
	\eU		uppercase till \eE
	\eE		end case modification

.fi
You can also embed newlines directly in your strings, i.e. they can end on
a different line than they begin.
This is nice, but if you forget your trailing quote, the error will not be
reported until
.I perl
finds another line containing the quote character, which
may be much further on in the script.
Variable substitution inside strings is limited to scalar variables, normal
array values, and array slices.
(In other words, identifiers beginning with $ or @, followed by an optional
bracketed expression as a subscript.)
The following code segment prints out \*(L"The price is $100.\*(R"
.nf

.ne 2
    $Price = \'$100\';\h'|3.5i'# not interpreted
    print "The price is $Price.\e\|n";\h'|3.5i'# interpreted

.fi
Note that you can put curly brackets around the identifier to delimit it
from following alphanumerics.
Also note that a single quoted string must be separated from a preceding
word by a space, since single quote is a valid character in an identifier
(see Packages).
.PP
Two special literals are _\|_LINE_\|_ and _\|_FILE_\|_, which represent the current
line number and filename at that point in your program.
They may only be used as separate tokens; they will not be interpolated
into strings.
In addition, the token _\|_END_\|_ may be used to indicate the logical end of the
script before the actual end of file.
Any following text is ignored, but may be read via the DATA filehandle.
(The DATA filehandle may read data only from the main script, but not from
any required file or evaluated string.)
The two control characters ^D and ^Z are synonyms for _\|_END_\|_.
.PP
A word that doesn't have any other interpretation in the grammar will be
treated as if it had single quotes around it.
For this purpose, a word consists only of alphanumeric characters and underline,
and must start with an alphabetic character.
As with filehandles and labels, a bare word that consists entirely of
lowercase letters risks conflict with future reserved words, and if you
use the
.B \-w
switch, Perl will warn you about any such words.
.PP
Array values are interpolated into double-quoted strings by joining all the
elements of the array with the delimiter specified in the $" variable,
space by default.
(Since in versions of perl prior to 3.0 the @ character was not a metacharacter
in double-quoted strings, the interpolation of @array, $array[EXPR],
@array[LIST], $array{EXPR}, or @array{LIST} only happens if array is
referenced elsewhere in the program or is predefined.)
The following are equivalent:
.nf

.ne 4
	$temp = join($",@ARGV);
	system "echo $temp";

	system "echo @ARGV";

.fi
Within search patterns (which also undergo double-quotish substitution)
there is a bad ambiguity:  Is /$foo[bar]/ to be
interpreted as /${foo}[bar]/ (where [bar] is a character class for the
regular expression) or as /${foo[bar]}/ (where [bar] is the subscript to
array @foo)?
If @foo doesn't otherwise exist, then it's obviously a character class.
If @foo exists, perl takes a good guess about [bar], and is almost always right.
If it does guess wrong, or if you're just plain paranoid,
you can force the correct interpretation with curly brackets as above.
.PP
A line-oriented form of quoting is based on the shell here-is syntax.
Following a << you specify a string to terminate the quoted material, and all lines
following the current line down to the terminating string are the value
of the item.
The terminating string may be either an identifier (a word), or some
quoted text.
If quoted, the type of quotes you use determines the treatment of the text,
just as in regular quoting.
An unquoted identifier works like double quotes.
There must be no space between the << and the identifier.
(If you put a space it will be treated as a null identifier, which is
valid, and matches the first blank line\*(--see Merry Christmas example below.)
The terminating string must appear by itself (unquoted and with no surrounding
whitespace) on the terminating line.
.nf

	print <<EOF;		# same as above
The price is $Price.
EOF

	print <<"EOF";		# same as above
The price is $Price.
EOF

	print << x 10;		# null identifier is delimiter
Merry Christmas!

	print <<`EOC`;		# execute commands
echo hi there
echo lo there
EOC

	print <<foo, <<bar;	# you can stack them
I said foo.
foo
I said bar.
bar

.fi
Array literals are denoted by separating individual values by commas, and
enclosing the list in parentheses:
.nf

	(LIST)

.fi
In a context not requiring an array value, the value of the array literal
is the value of the final element, as in the C comma operator.
For example,
.nf

.ne 4
    @foo = (\'cc\', \'\-E\', $bar);

assigns the entire array value to array foo, but

    $foo = (\'cc\', \'\-E\', $bar);

.fi
assigns the value of variable bar to variable foo.
Note that the value of an actual array in a scalar context is the length
of the array; the following assigns to $foo the value 3:
.nf

.ne 2
    @foo = (\'cc\', \'\-E\', $bar);
    $foo = @foo;		# $foo gets 3

.fi
You may have an optional comma before the closing parenthesis of an
array literal, so that you can say:
.nf

    @foo = (
	1,
	2,
	3,
    );

.fi
When a LIST is evaluated, each element of the list is evaluated in
an array context, and the resulting array value is interpolated into LIST
just as if each individual element were a member of LIST.  Thus arrays
lose their identity in a LIST\*(--the list

	(@foo,@bar,&SomeSub)

contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub.
.PP
A list value may also be subscripted like a normal array.
Examples:
.nf

	$time = (stat($file))[8];	# stat returns array value
	$digit = ('a','b','c','d','e','f')[$digit-10];
	return (pop(@foo),pop(@foo))[0];