How naive I was to loop through looking for something already given...

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

my $data = 0b1111_1111_1111;
my $offset = 5;

my $mask = hex 'F' x ($offset % 4 == 0 ? $offset / 4 : $offset / 4 + 1
+);

printf("%b\n",$data ^ $mask);
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