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";
[download]

Prints:

2147483648
4294967296
1
4294967296
[download]

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