#!/usr/bin/env perl
use strict;
use warnings;

local $/ = undef;
my $string = <DATA>;

while ($string =~ m{
                     \b            #word boundary
                     (\w+)         #word (saved in $1)
                     \b            #word boundary
                     (?=      #Look ahead, but do not "eat up", all the following
                        (.*)        #Anything (and save in $2)
                         \b         #word boundary
                         \1         #same word we already found
                         \b         #word boundary
                     )
                  }sigx){    #allow . to match \n, case insensitive, progressive matching
    my ($word, $between) = ($1, $2);
    print "Between copies of '$word':\n";
    print "="x35, "\n";
    print "$between\n";
    print "="x35, "\n\n";
}


__DATA__
The quick brown fox jumps over the lazy dog,
A fox stands by my dog over there.
This text is over now.