how to update row by replacing substitution

[srey bo]

Hi,
This is my sample data (tbl):
id firstname lastname
1 aaabbb aaacccc
2 aaaddd aaaeeee
3 aaafff aaagggg

I want to replace aaabbb by bbb, aaaddd by ddd, and so on.
so i use sql statement like this in perl:

my $sth=$dbh->prepare("update tbl set firstname=(firstname =~ s/^aaa//) where id=?");
$sth->execute($id);

It doesn't work.
Have any idea?
please help me.....