As the other answer said, none of these functions receive any arguments, nor do they return anything meaningful.

In Perl, to pass arguments to a function, you put them in parentheses after the function name, a la:

my @nums    = 1 .. 10;
my @results = evens(@nums);
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And within the functions, to receive arguments, you pull them out of the @_ array.

sub evens {
  my (@nums) = @_;
  ...;
}
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To return something, you either use the return keyword, or make sure that the thing you want to return is the last statement executed in the sub.

sub this_sub_returns_1 {
  return 1;
  2;
}

sub this_sub_returns_2 {
  1;
  2;
}
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Here are even and square rewritten to actually take arguments and return values. There's also even_better and square_better which do the same thing but more concisely. (Whether they are "better" is a judgement call. I prefer them though.)

use strict;
use warnings;

# Write a subroutine (&evens) that recieves an array of numbers and re
+turns 
# an array containing only even numbers Give an example of calling the
+ 
# subroutine.

sub even
{
    my (@nums) = @_;
    my @evens;
    foreach my $number (@nums)
    {
        if ($number % 2 == 0)
        {
            push @evens, $number;
        }
    }
    return @evens;
}

print "Results for even(1..10)\n";
print "$_\n" for even( 1 .. 10 );

sub even_better
{
    grep { $_ % 2 == 0 } @_;
}

print "Results for even_better(1..10)\n";
print "$_\n" for even_better( 1 .. 10 );

# Write a subroutine(&squares) that recieves an array of numbers and 
# squares each number in the array. Note: nothing is returned. 

sub square
{
    foreach my $number (@_)
    {
        $number *= $number;
    }
    return;
}

print "Results for square(1..10)\n";
my @nums = 1 .. 10;
square(@nums);
print "$_\n" for @nums;

sub square_better
{
    $_ *= $_ for @_;
}

print "Results for square_better(1..10)\n";
my @nums_better = 1 .. 10;
square_better(@nums_better);
print "$_\n" for @nums_better;
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