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

##</code><code>##

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

##</code><code>##

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