Perl Reference, Quick Reference

Scalar references:

$ra = $a;	# reference to scalar
$$ra = 2;	# dereference scalar- ref
$ra = 1.6;	# reference to constant scalar

Array references:

$rl = @l;	# reference to existing
$rl = [1,2,3];	# reference to anonymous array
push (@$rl, "a");	# Dereference
print $rl->[3]	# 4th element of array pointed to by $rl

Hash references:

$rh = %h;	# reference to hash
$rh = {"laurel" => "hardy", "romeo" => "juliet"};	# ref to anon-hash
print keys (%$rh);	# Dereference
$x = $rh->{"laurel"};	# Arrow notation to extract single element
@slice = @$rh{"laurel","romeo"};	# Hash slice

Code references:

$rs = &foo;	# reference to existing subroutine foo
$rs = sub {print "foo"};	# reference to anonymous subroutine # (remember the semicolon at the end)
&$rs();	# dereference: call the subroutine

Generalized dereferences. Any code inside a block yielding a reference can be dereferenced:

@a = @{foo()};

# dereference the array reference
# returned by foo()

References gotchas. All the examples below are wrong. Always use -w in developing and testing.

@foo = [1,3,4];	# Assigning an array-ref to an array # Use parentheses instead.
%foo = {"foo" => "bar"};	# Assigning a hash-ref to a hash. # Use parentheses instead.
$foo = ($a, @b);	# Identical to $foo = ($a, @b) # Assiging an enumerated list to a # scalar yields the last element (so, # $foo gets @b). Use [ ] if you need # an array reference

Lists do not nest like this:
@foo = (1, 3, ("hello", 5), 5.66);

For nesting, make the third element a reference to an anonymous array:
@foo = (1, 3, ["hello", 5], 5.66);

An example of a nested data structure (a hash of array of hashes):

$person = { # Anon. hash "name" => "John", # '=>' is an alias for a comma "age" => 23, "children" => [ # Anonymous array of children { "name" => "Mike", "age" => 3, }, { "name" => "Patty","age" => 4 } ] };
print $person->{age};	# Print John's age
print $person->{children}->[0]->{age};	# Print Mike's age
print $person->{children}[0]{age};	# Print Mike's age,omitting # arrows between subscripts

To pretty-print $person above:

use Data::Dumper;
Data::Dumper->Dumper($person);

# Or,

require dumpVar.pl;
main::dumpValue($person);

Exception Handling

die throws an exception, and eval traps it. Error string is found in $@. The following code has the possibility of two run-time errors:

eval {
$c = $a / $b;
die "Denominator cannot be negative" if ($b < 0);
};
print "Run-time error: $@";

$@ can be "Illegal division by zero" or "Denominator

cannot be negative".

Meta-Information

Call-stack information. Use caller() to find out who's calling this subroutine:

($package, $file, $line) = caller();

List of a package's global variables. For a package Foo, %Foo:: contains the symbol table, whose keys are names of global identifiers in that package and whose values are typeglobs.

Find out what a reference contains. ref($r) returns undef if $r is an ordinary scalar, "SCALAR" if it is a reference to a scalar (similarly "ARRAY," "HASH," "CODE," and "REF") or the name of a package, if $r is a blessed object reference.

Object information:
$obj->isa("Foo"); # returns true if $obj inherits from Foo
$obj->can("bar"); # returns true if it supports method "bar"