Thanks. Either the code as posted on Rosetta has a typo // instead of || or I misread it. Once I realized that the // should be read as || it all made sense. Thanks again. Those replies were quicker than my thinking process. :(!

No. || and // are two distinct operators. The first one checks whether its first argument is true, the second one whether it's defined; if not, they return the second argument. The difference becomes apparent with defined false values like 0:

choroba@still ~ $ perl -wE 'say 0 || 1'
1
choroba@still ~ $ perl -wE 'say 0 // 1'
0
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map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]