There will be two issues at play -
What CAN the employer do,
What is it WORTH the employer doing.
Whether or not the employee can "jump ship" will be in the terms written in the employment contract (once letter of offer is signed, you are an employee)
Most of the low - mid level contracts I see are such that for non-confirmed staff, notice is anywhere between 1 week and 1 month.
So, if the employee jumps ship with less than the notice period - by rights, they have to pay in lieu of notice as per the contract.
By left, however - it's something pretty difficult for the employer to be worthwhile pursuing. I don't think employment is covered by small claims court - so you have to engage a lawyer to chase it through the court system.
The expense of engaging a lawyer, court costs etc etc will be way more than you could expect to recover. Of course - if you are right, you will be awarded costs. BUT my understanding is that it is rare for 100% of costs to be awarded - figures like 60% are more common. By the time that you factor in even 40% of costs, with the uncertainty of winning - against someone that doesn't want to work with you, it doesn't seem like much of a winning proposition to me.
Now of course - the employee needs to remember that Singapore is a very small place still, and you never know when you will run across someone again, so you don't want to be burning bridges.
By the same token - no employer wants the reputation that they sue and "bully" employees (yes, I know, employer is in the right, so it's not really bullying, but still, they have a power differential and who wants to work for someone like that?)
The upshot? As an employee - I would be as honest with the employer as possible, while still not going to work, as the employer - I would KPKB but ultimately LLST
Behold the tortoise! He only makes progress when he sticks his neck out.
All that's required for evil to triumph is that good men do nothing