Reading the perlre man page will put hair on your chest (and for those already endowed, closer reading can cause graying). Here's a relevant nugget:

  The bracketing construct "( ... )" creates capture
  buffers.  To refer to the digit'th buffer use \<digit>
  within the match.  Outside the match use "$" instead of
  "\".  (The \<digit> notation works in certain circum­
  stances outside the match.  See the warning below about \1
  vs $1 for details.)  Referring back to another part of the
  match is called a backreference.
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In other words, use "backslash-digit" to refer to a paren'ed chunk while you're still in the left side of the expression, use "dollar-digit" to place a chunk in the replacement pattern.