use constant BIT0 => 0;
use constant BIT1 => 1;
use constant BIT2 => 2;
use constant BIT3 => 4;
#...

##</code><code>##

my %bitNames = (
    BIT0 => 0,
    BIT1 => 1,
    BIT2 => 2,
    BIT3 => 4,
    BIT4 => 8,
#...
);

##</code><code>##

my $flag=0; # or value from your api
vec($flag, $bitNames{BIT0}, 1)=$newBit;
# you can also use vec to retrieve the old bit values:
print "Value at position BIT3 is:",vec($flag, $bitNames{BIT3},1),"\n";