You could probably use Pl/Python. Python has base64 module and
urllib module which can deal with all URL strings I am aware of.
Pl/Perl would probably do as well.
select (utl_raw.cast_to_varchar2 (utl_encode.base64_decode( utl_raw.cast_to_raw (regexp_replace ( ( 'eyJhbGciOiJSUzI1NiIsImtpZCI6InBhLWFwaS1zaWduIn0.eyJzY29wZSI6W10sInN1YiI6Imp1bGllQHNhcHBoaXJlY29udmV5YW5jaW5nLm5ldC5hdSIsImNsaWVudCI6InB4YV9hY19zZXR0c3BsdXMiLCJ1c2VybmFtZSI6Imp1bGllQHNhcHBoaXJlY29udmV5YW5jaW5nLm5ldC5hdSIsImV4cCI6MTU1NDA4NzU2N30.OZwVETCP+HUDsSkudCfa52eQT2Fyt6BgefFpYr5xpJROjZJeVnXtzsU6MS8aHauqyyOwQSz8iG6OC2YjYgrGiZ8YTwTh1uHptNjicyCFmCkh++d0oGdK78ZOKIAvuslW3GX4V13hPUJADzGLkyO8hy01YF9Id4+7GrvmnRRkwPO7C2eUvKTB8pxDAA94Jpw74s4BiNTmm1Uv+56AXqdI8m2KFkLNwGoUR3OSNvsifl5uxBsGGmP4wbgNe/Dy1q2UsL2eBAW/GuWryHGCRz1/PR2jIV1v21ClQ8YLiFAzM3YxHb8m2ngzRynKof+Nw3nZvuafRQ16MV5THz0ahGFYLA'
-- Mladen Gogala Database Consultant Tel: (347) 321-1217 https://dbwhisperer.wordpress.com