# declare a function
sub my_function_name {
  # Your input parameters will be in the array @_...
  my $first_arg = shift(@_);
  my @remaining args = @_;

  # Then do whatever you want here...
  print "Hello, world\n";

  # And return data
  return "Whatever you want to\n";
}

# And then you call it like this:
my_function_name("First argument", "the", "rest", "of", "them");