The parenthesis control the execution order so the 1 gets shifted left 31 places, then the result is multiplied by 2. Note that this is a trick to get around Perl's internal representation for integers (on many platforms) as 32 bit quantities. Consider:

print 1<<31, "\n";
print 2 * (1<<31), "\n";
print 1<<32, "\n";
print 2**32, "\n";
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Prints:

2147483648
4294967296
1
4294967296
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Although it's not clear to me why 2**32 was not used - your code seems rather odd to me.

Or consider this which doesn't use multiplication (but only works on a 32 bit machine.)

$ perl -le'print ~0 + 1'
4294967296
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I can imagine that 2**32 might not be optimised into anything cheaper. 2.1**3.1 would always be expensive for example and you don't expect an interpreter to be looking for a bit-shift optimisation because that would slow ** down even more if one didn;t exist. By comparison, << is a very cheap operator when translated down to machine code level and ** is one of the dearest.
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^M Free your mind!