Ok, tonight's trick simply illustrates the principle of replacing the standard '/' separator in substitutions with other characters. It's a simple and well known trick, but can be used pretty effectively in some instances. Here goes:

$_ = "Sello World\n";

# think about alternative separators
s'S'H's;
print;

# Deparse and compare the difference in the next two
s=>o=>O=>o;
print;

s,o,O,o;
print;

# and now, for the mystery...
sub s { ("x","y","z"); }
s()[0];
print;
[download]

The 'mystery' has nothing to do with mixing separators or that you can use 4 instead of three, that's pretty simple stuff... it's why it replaces anything at all, much less the character it does. Happy obfu-writing!