my $palindrome_re = qr{
    \b
    (?<pal>          # A palindrome is a sandwich...
        (?<char>\w)  # starting with white bread...
        (?:          # filled with either...
            \w?      #   peanut butter (maybe)...
        |            # or...
            (?&pal)  #   another sandwich...
        )
        \k<char>     # topped off with white bread!
    )
    \b
}x;

@ARGV = 'words';
/$palindrome_re/ and print while <>;