There are a few bit manipulation tricks worth knowing about when you are dealing with masking blocks of bits:

#!/usr/bin/env/perl
use strict;
use warnings;

my $oValue = 0xAAAA;
my $setBit = 1 << 5;
my $mask = $setBit - 1;
my $setBits = $oValue | $mask;
my $resetBits = $oValue & ~$mask;

printf "orig: %016b\n", $oValue;
printf " bit: %016b\n", $setBit;
printf "mask: %016b\n", $mask;
printf " set: %016b\n", $setBits;
printf " clr: %016b\n", $resetBits;
[download]

Prints:

orig: 1010101010101010
 bit: 0000000000100000
mask: 0000000000011111
 set: 1010101010111111
 clr: 1010101010100000
[download]

Note that ~ generates the 1s complement (inverts each bit) of its operand.