You've read all the perldocs, tutorials and can quote verbatim from the Camel book. Even so you come across sections that don't make immediate sense or you'll think "I'm never going to use that!".

Like in perlre:

 (??{ code })
    ...
    This is a "postponed" regular subexpression.  The
    "code" is evaluated at run time, at the moment this
    subexpression may match. The result of evaluation is
    considered as a regular expression and matched as if
    it were inserted instead of this construct.
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"What would I use that for?"

Well, I came across a use yesterday. I find that B<some bold text> is easier to read than <b>some bold text</b>. Hence I wanted to use that on my website to make writing their web pages easier for my kids & wife. I didn't need the full POD syntax, so here it is: the (over)simplified one-line POD-2-HTML text marker.

   1 while s!([BUI])(<+)\s*(.*?)\s*(??{'>' x length $2})!<$1>$3</$1>!;
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It translates:

   "B<<< I<< U<bold underlined text> >> >>>"
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to

  <B><I><U>bold underlined text</U></I></B>
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(I know that in this case the HTML version is actually more readable than the bracketed version, but I just wanted to show that it handles stacked markup.)

Regex explanation:

my $pod2html = qr/
  ([BUI])                  # start of formatting pattern
  (<+)                     # collect opening braces
  \s*(.*?)\s*              # collect text, without border space
  (??{'>' x length($2)})   # match same number of closing braces
/x;
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By using a hash lookup, this is easily expanded to include all kinds of translations.