/
    (                            # capture group
        ^                        # match begin of string
        .*                       # match everything
        \z                       # match end of string
    )
    (?                           # if
        (?{ $+ eq reverse $+ })  # $+ (last captured group) is palindrome
                                 # then match nothing (succeed)
    |                            # else
        (?!)                     # fail on matching nothing (fail the regex)
    )
/xs                              # x for readability, s to make . match \n

# Note: can match empty string