That's just screaming for factorisation. ;-)

/^a(|b(|b(|r(|e(|v(|i(|a(|t(|i(|o(|n)))))))))))$/
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Or, to possibly make the abbreviation stand out a bit more at the expense of some readability:

/^a(b(b(r(e(v(i(a(t(i(o(n|)|)|)|)|)|)|)|)|)|)|)$/
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(The non-toy solution probably involves Text::Abbrev.)