Mini-Tutorial: Dereferencing Syntax

[ikegami]

Whenever you can use the name of a variable, you can use a block that evaluates to a reference instead. For example, the following are valid syntax for specifying an array:

@NAME    # If you have the name      e.g. @array
@BLOCK   # If you have a reference   e.g. @{ $ref }

This is the "circumfix syntax" illustrated in the table below.

An alternative syntax colloquially known as the "arrow notation" is easier to read, but requires Perl 5.20+ in some cases. This is the "postfix syntax" illustrated in the table below.

		Direct	Using References
			Circumfix Syntax[1]	Postfix Syntax
scalar	itself	$s	${$sr}	$sr->$*[3]
array	itself	@a	@{$ar}	$ar->@*[3]
	element	$a[0]	${$ar}[0]	$ar->[0]
	slice	@a[0,1,2]	@{$ar}[0,1,2]	$ar->@[0,1,2][3]
	index-value slice[2]	%a[0,1,2]	%{$ar}[0,1,2]	$ar->%[0,1,2][3]
	last index	$#a	$#{$ar}	$ar->$#*[3]
hash	itself	%h	%{$hr}	$hr->%*[3]
	element	$h{'a'}	${$hr}{'a'}	$hr->{'a'}
	slice	@h{'a','b','c'}	@{$hr}{'a','b','c'}	$hr->@{'a','b','c'}[3]
	key-value slice[2]	%h{'a','b','c'}	%{$hr}{'a','b','c'}	$hr->%{'a','b','c'}[3]
code	call	func(@args)
	call (ignored prototype)	&func(@args)	&{$cr}(@args)	$cr->(@args)
	call (inherited @_)	&func	&{$cr}	$cr->&*[3]
glob	itself	*glob	*{$gr}	$gr->**[3]
	slot	*glob{ARRAY}	*{$gr}{ARRAY}	$gr->*{ARRAY}[3]

1. The curly brackets around $sr, $ar, $hr, $cr and $gr are optional when the contents of the curlies is simply a scalar.
2. Requires Perl 5.20+.
3. Requires Perl 5.24+. Available in Perl 5.20+ by adding both use feature qw( postderef ); and no warnings qw( experimental::postderef );, or by adding use experimental qw( postderef );. This is safe because the then-experimental feature was accepted into Perl without change.

See also:

• References Quick Reference
• perlref
• perlreftut
• perldsc
• perllol

[tobyink]

It's also worth noting that the following are two completely different things.

@hash{'key1', 'key2'}

$hash{'key1', 'key2'}

The first one is basically a shorthand for:

($hash{'key1'}, $hash{'key2'})

The second one is:

$hash{join($;, 'key1', 'key2')}

... where $; is the subscript separator, which defaults to \034.

[jdporter]

And similarly (though not identically, certainly) for arrays.

[ikegami]

The difference between hash lookups and hash slices has nothing to do with references or how to use them. I shall not add such a note.

[ikegami]

Updated for Perl 5.20.

...And previously missing $ar->$#* was added to both Perl and this document.

[Anonymous Monk]

https://metacpan.org/pod/release/BINGOS/perl-5.19.6/pod/perlref.pod#Postfix-Dereference-Syntax

[Anonymous Monk]

Not mentioned:   $$foo as shorthand for $foo->.

[ikegami]

$$foo is not a shorthand for $foo->.

>perl -E"$foo=\'foo'; say $$foo;"
foo

>perl -E"$foo=\'foo'; say $foo->;"
syntax error at -e line 1, near "->;"
Execution of -e aborted due to compilation errors.

You're probably thinking of the equivalency of $$foo[...] and $foo->[...] and of $$foo{...} and $foo->{...}, but that's already mentioned.

[DrHyde]

Huzzah for the slice syntax!