plpgsql: PERFORM vs. SELECT INTO (PERFORM not setting FOUND variable?)

Hi,

I'm probably misintepreting the documented equivalence of PERFORM and
SELECT INTO as far as the special variable FOUND is concerned, but the
following 2 definitions do not seem to produce the same result.

create table blup ( t1 text, t2 text );

create function blup_unique2 (text,text) returns boolean as 'begin
perform (select 1 from blup where t1=$1 or t1=$2 or t2=$1 or t2=$2 or
$1=$2 limit 1); return NOT FOUND; end' LANGUAGE plpgsql;

create function blup_unique3 (text,text) returns boolean as 'declare x
record; begin select into x 1 from blup where t1=$1 or t1=$2 or t2=$1 or
t2=$2 or $1=$2 limit 1; return NOT FOUND; end' LANGUAGE plpgsql;

The first will always produce false, i.e. apparently the subquery used
by PERFORM will not set the variable FOUND as expected. Is this correct?

(btw.: I'm trying a few ways to ensure that all values in both t1 and t2
are unique:
alter table blup add constraint check (blup_unique3(t1,t2));
- perhaps there are more elegant ways, any suggestions?)

Regards,
Marinos