if [ $YOU = 'frog' ]
   then
     export OUSER=TNN
     export CAR=SWs
     export O_PREFIX=ss
     export OS=corvette

fi

##</code><code>##

my $newUser ='dummy';
while(<SETORA>)
{
s/O_PREFIX=\w+/O_PREFIX=$newUser/ if /O_PREFIX=/;
print outputfile
}