use strict;
use warnings;

my $text = do { local $/; <DATA> };

$text =~ s/(MONKS[\s\(\)]+{.*?,\s*?)2329(\s*?,[^}]*})/${1}9999${2}/sg;

print "$text";

__DATA__
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
}

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

And the output -

MONKS ()
{
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null,
+'$8',9999,$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
}

MONKS ()
{
echo "insert into monks values ('$1',sysdate,null,null,null,'OP',null,
+'$8',9999,$3);" >> $OPTHREE
}
[download]