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/(^.*\z)(?(?{ $+ eq reverse $+ })|(?!))/s

/
    (                            # capture group
...
/xs                              # x for readability, s to make . matc
+h \n

# Note: can match empty string

rule palindrome {
    $foo := (\w+) ::: { fail if $foo ne reverse $foo }
}

# Note: doesn't match empty string

/(.+)(??{ reverse $+ })/s

/
    (.+)                          # match one or more characters
...
/xs

# Note: doesn't match empty string or single character string

rule palindrome {
    $foo := (\w+) <{ quotemeta reverse $foo }>
}