Check this out. It finds the reoccuring phrases "of", "on", "Leonardo da Vinci", "da Vinci", and "Vinci".

$_ = <<"...";
Leonard of Quirm, a character in the Discworld series of novels,
is based largely on Leonardo da Vinci.

Leonardo da Vinci died at Clos Lucé, France, on 2nd May, 1519.
...

# Normalize the whitespace
s/\s+/ /g;

my $RX = qr/
# NODE                     EXPLANATION
# --------------------------------------------------------------------
+--
  \b                     #   the boundary between a word char (\w) and
                         #   something that is not a word char
# --------------------------------------------------------------------
+--
  (                      #   group and capture to \1:
# --------------------------------------------------------------------
+--
    \w+                  #     word characters (a-z, A-Z, 0-9, _) (1 o
+r
                         #     more times (matching the most amount
                         #     possible))
# --------------------------------------------------------------------
+--
    (?:                  #     group, but do not capture (0 or more
                         #     times (matching the most amount
                         #     possible)):
# --------------------------------------------------------------------
+--
      \s+                #       whitespace (\n, \r, \t, \f, and " ")
                         #       (1 or more times (matching the most
                         #       amount possible))
# --------------------------------------------------------------------
+--
      \w+                #       word characters (a-z, A-Z, 0-9, _) (1
                         #       or more times (matching the most
                         #       amount possible))
# --------------------------------------------------------------------
+--
    )*                   #     end of grouping
# --------------------------------------------------------------------
+--
  )                      #   end of \1
# --------------------------------------------------------------------
+--
  \b                     #   the boundary between a word char (\w) and
                         #   something that is not a word char
# --------------------------------------------------------------------
+--
  .+?                    #   any character except \n (1 or more times
                         #   (matching the least amount possible))
# --------------------------------------------------------------------
+--
  \b                     #   the boundary between a word char (\w) and
                         #   something that is not a word char
# --------------------------------------------------------------------
+--
  \1                     #   what was matched by capture \1
# --------------------------------------------------------------------
+--
  \b                     #   the boundary between a word char (\w) and
                         #   something that is not a word char
/xms;

while ( /$RX/sg ) {
    pos() = $-[0] + 1;
    print "<$1>\n";
}
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