Functions are used when you want to reuse a block of code. You call or invoke a function by its name and passing arguments if the functions accepts any.

In Perl, @_ contains a list of arguments that was passed to the function. This list will have scalars, arrays and hashes flattened into one.

The example below will show how to call a function and define a function. Note that only a scalar was passed to the function and $_[0] contains that value;

#!/usr/bin/perl

use strict;
use warnings;

my $price = 12.99;

calculate_total($price);

sub calculate_total
{
    my $p = $_[0];

    print "You must pay ", $p;
}

When you run the following script, Perl will give you a warning that says “Odd number of elements in hash assignment” and $d is undef. This is because the hash is greedy and gets all the values from @_.

#!/usr/bin/perl

use strict;
use warnings;

my %pizza = (plain => 9.99, pepperoni => 10.99);
my $drink = 3.99;

display_price(%pizza, $drink);

sub display_price
{
    my (%p, $d) = @_;

    foreach my $i (keys %p)
    {
        print $p{$i}, "\n";
    }
    print $d, "\n";
}

Instead, you will want to pass the scalar first and then the hash. This will produce what you expected.

#!/usr/bin/perl

use strict;
use warnings;

my %pizza = (plain => 9.99, pepperoni => 10.99);
my $drink = 3.99;

display_price($drink, %pizza);

sub display_price
{
    my ($d, %p) = @_;

    foreach my $i (keys %p)
    {
        print $p{$i}, "\n";
    }
    print $d, "\n";
}

This example demonstrates how hash references are passed to functions.

#!/usr/bin/perl

use strict;
use warnings;

my %pizza = ( plain => { medium => 7.99, large => 9.99}, 
              pepperoni => { medium => 9.99, large => 10.99}, 
              supreme => { medium => 10.99, large => 12.99}, 
              veggie => { medium => 9.99, large => 10.99}
            );
my %order = ( plain => { large => 2},
              pepperoni => { large => 2}
            );

calculate_total(\%pizza, \%order);

sub calculate_total
{
    my ($p, $o) = @_;

    my $total = 0;
    foreach my $style (keys %$o) {
        print "$style \n";
        foreach my $size (keys %{ $o->{$style} }) {
            my $pies = $o->{$style}{$size};
            print "$size: $pies pies\n";

            $total += ($p->{$style}{$size} * $pies);
        }
    }
    print "Your total is ", $total, "\n";
}

If you want to return $total to the main, you will have to use a return statement inside the function.

return $total;