I'll pick one nit with merlyn's example:

  $a = 3 # no semi needed here
  if ($a == 5) { print "b" }
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The problem is that both   $a = 3 and   $a = 3 if ($a == 5) are legal statements in the Perl grammar. Who is to say that the author didn't mean:   $a = 3 if ($a == 5); { print "b" } This is one of the perils with error recovery in compilers, and a particular problem with Perl.