You are getting stuffed up by the quoting here. You should bear in mind in the first instance that the first argument to split is a regular expression, and it is often prefered use the // rather than quotes. In your first attempt you are asking to split on anything or anything ('|' being the regex alternation character), in your second you are basically getting the same thing because the '\' is being eaten in the double quoted context (i.e. escaping the '|') and the third is correct because you are now escaping the '\' so you get a literal '\|'. You probably want to either use single quotes or the // for clarity.

use warnings;
use strict;
                                                                      
+            
my $str = "lhs|rhs";
                                                                      
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print "\n" . join " ", split '\|', $str;
print "\n" . join " ", split /\|/, $str;
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/J\