Read the comments in number order:

## which are finally assigned to the array                      ## 4 #
+#
my @newest = (
    ## This runs next, and sorts the directories                 ## 2 
+##
    ## from most recent to least recent
    sort{ -M $a <=> -M $b } 
    
    ## this runs first, generates a list of files/dirs in root   ## 1 
+##
    ## which it then reduces to just a list of directories
    grep -d, glob '/*'
)[0..19]; ## then this "list slice" selects the 20 most recent  ## 3 #
+#
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It is equivalent to:

my @allFilesnDirs = glob '/*';

my @allDirs = grep -d, @allFilesnDirs;

my @allDirsSortByDateTime = sort{ -M $a <=> -M $b } @allDirs;

my @newest20 = @allDirsSortedByDateTime[ 0..19 ];
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