The most important point is that split interprets the first argument as a regex. So split '.', split "." and split /./ are exactly the same.

The rest of your question is easy to answer if you just print out the string or regex that split sees:

$ perl -wle 'print "\."'
.
# so it's the same as /./

$ perl -wle 'print "\\."'
\.
# as a regex, matches a literal dot
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In the case of regexes, /\./ matches a literal dot, /\\./ a backslash followed by any character, /\\\./ a backslash followed by a literal dot and so on.

Update: There's also a special quoting for to create regexes, that avoids having to use excessive amounts of backslashes if you want to store a pattern in a variable:

my $regex = qr{\.}; # matches a literal dot

my @chunks = split $regex, 'dot.delimited.string';
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