Subroutines

Declaring

sub foo;

Defining

sub foo {
    my $arg1 = $_[0];
    ...
}

sub bar {
    my ($arg1, $arg2, $arg3) = @_;
    ...
}

• Subroutines cannot be nested
• Subroutines can be defined anywhere outside of a block

• Subroutines can recursively call themselves
  
• Subroutines are untyped
  

Arguments

• Arguments are contained within the special array @_ ; its elements are aliases (i.e. passed by reference) for the actual scalar parameters

my $arg1 = $_[0];

my ($arg1, $arg2, $arg3) = @_;

my $arg1 = shift;   # Remove and return the first argument

• Arguments are flattened and merged into a single array, as happens in a list context:
  

sub foo {
    # As called below, @_ will have the value ("one", "two", "three", "four", "five");
}

foo( ("one", "two"), "three", ("Four", "five") );

• To prevent array and hash arguments from being flattened, pass a reference to each:

sub foo {
    my ($str, $arr_ref, $hash_ref) = @_;
}

my $str = "hello";
my @arr = ("one", "two", "three");
my %hash = (one => 'apple', two => 'orange');
foo($str, \@arr, \%hash);                       # Pass references to the array and hash arguments

• Sample usage of parameters    Output

Return value

• A return statement can be used to exit the subroutine and to state the return value
• Can return either a list or a scalar
• By default, the return value is the last expression evaluated

• If a return statement does not specify a value, the subroutine returns an empty list in list context, the undefined value in scalar context, or nothing in void context

Calling

foo;
foo(arg1, ...);

&foo(arg1, ...);

• Parentheses for arguments (when calling a subroutine) are optional
• The leading ampersand is usually optional (and typically omitted)