Sometimes it's just a lot easier to use two regexes:

#!perl

use warnings;
use strict;

while (<DATA>) {
    if ($_ =~ /^MONKS/ .. /}/) { #Range/flip-flop operator
        s/,2329,/,9999,/;
    }
    print;
}
__DATA__
LOSER ()
{
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null,
+'$8',23,$3);" >> $OPTHREE
}

MONKS ()
{
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null
+'$8',2329,$3);" >> $OPTHREE
}

TEST ()
{
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null,
+'$8','$5',9,$2,$4,$3);" >> $OPTHREE
# the following line should remain unchanged
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null,
+'$8',2329,$3);" >> $OPTHREE
}
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Check your docs for the range operator and its 'flip-flop' (scalar) context if the '/^MONKS/ .. /}/' gives you trouble.