Can I use run Bitcoin Core in regtest mode in v0.8.1? I'm trying to run bitcoin-core v0.8.1 in regtest mode, but it doesn't work.
When placing regtest=1 in bitcoin.conf it starts to sync the mainnet.
I've not seen anything related in the code.

So I was wondering if there's any configuration different to run regtest mode in that version different from more recent ones.

When was regtest mode added ?
Regtest mode was introduced in version 0.9.0, via pull request #2632. That is why it doesn't work in 0.8.1, as you noticed. There is no other option to run regtest in that version, although testnet3 is supported by that version so you should still be able to use that if it suits your needs. Otherwise just upgrade to a newer version which will contain performance improvements, bugfixes and newer feature support :)