sub add {
  return shift() + shift();
}

##</code><code>##

sub add {
  return $_[0] + $_[1];
}
# ...or...
sub add {
  my ( $left, $right ) = @_;
  return $left + $right;
}

##</code><code>##

my $size = length( $some_string );

##</code><code>##

sub mylength {
  my $string = shift;
  my $size = $string =~ tr///c; # Don't worry how it works.
  return $size;
}

# Invoke it like this:
my $length = mylength("Hello world.");

# $length now holds 12.