$_ not set in while <>

[pidloop]

Oh great Monks, I have a simple (I think) question of understanding. I want to read a file until I find a line containing a pattern. I tried this:

while (<> and !/pattern/) {}

but it complained about $_ undefined in m// match. So I then made the assignment to $_ explicit like so and it works great:

while (defined($_ = <>) and !/pattern) {}

I am surprised by the need of doing this and would like to understand why. Thank you all!