Dyamic updates of NEW with pl/pgsql

How can a pl/pgsql trigger change the
values of dynamic fields in NEW record ?

By "dynamic" I mean that the field name
is a variable in the trigger context.

I've been told it's easy to do with pl/perl but
I'd like to delive a pl/pgsql solution to have
less dependencies.

Thanks in advance.

--strk; 
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