my $new_contents = my_test($old_content);
sub my_test {
  my $long_content = $_[0];
 # do some stuff here
  return $long_content;
}
##</code><code>##

$main_vars{content} = $old_content;
my $new_contents = my_test();
$old_content = $main_vars{content};
sub my_test {
 # do some stuff here with $main_vars{content}
}