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 Most of the power comes from implicit smart matching:
 
    when($foo)
...
is exactly equivalent to
 
    when($_ ~~ $foo)

#  $a $b Type of Match Implied Matching Code
# ====== ===== ===================== =============
...
#  Array Regex array grep grep /$b/, @$a

 given (@a) {
    when (/abc/) {}
}