simple and to the point

If you want some different calculations, like having free memory equals free+buffered+cached, then you need to make a couple of changes:

add
my $mem_buff = $sysctl->{“vfs.bufspace”};

and change:

#$free_memory = $mem_inactive + $mem_cache + $mem_free + $mem_buff;
$free_memory = $mem_cache + $mem_free + $mem_buff;