Can anyone suggest a Perl script that will check the number of files in a folder and delete the oldest two files, if there are more than 20 files present in the folder? Somebody has kindly given me the following bit of code:

opendir my $d, $dir; 

my @f = sort { -M $b <=> -M $a } readdir $d; 

unlink @f[-0,1] if @f > 20;
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Can anyone please expand this? Perhaps a detailed explanation. Or suggestions for improvement.