The purpose of the one-liner is to find the the "latest" version of the custlist_*.xls file from among multiple versions in directories and sub-directories, for example:

./c/custlist_v1.003.xls
./c/custlist_v2.001.xls
./d/b/custlist_v1.001.xls
./d/custlist_v1.002.xls

Let's decompose the one-liner to the big steps:

• find . -name custlist\* -- find the files matching the target pattern
• ... | perl -ne '...' -- run perl, with the input wrapped around in a while loop so that each line in the input is set in the variable $_
• $path = $_; s?.*/??; $name = $_; -- save the full path in $path, and cut off the subdirectory part to get to the base name of the file and save it in $name
• $map{$name} = $path; -- build a mapping of $name to $path
• ++$c; -- we count the elements, to use it later
• (sort(keys(%map)))[$c-1] -- sort the keys of the map, and get the last element, which is custlist_v2.001.xls in this example
• END { print $map{$last} }' -- at the end of all input data, print the path of the latest version of the file

Even if the latest version of the file appears multiple times in the directories, the one-liner will print only one of the paths. This could be fixed though if needed.