I work in this scenario (until three weeks when finally we move from Solaris to Linux, yay!), where the dev box is 5.6.0 while the old live server is 5.004.
It's not so bad, I guess, because you're aware of it. I personally was bitten by using 'mkdir' with only one argument and doing things like 'print for @daddy'. But it seems you're developing new code, so if you already have those things in mind and try to keep to the lowest common denominator.