This is why question content and formatting is so important: my expression was built for

03:20:21 - PID: 11500 - kernel bitness: 64

but the term you are trying to match is

The kernel bitness:   64

My expression failed because your real input contained more spaces. The following modified version matches the new provided input:

print $str =~ /kernel bitness:\s*(\d*)/;

By adding the * modifier to \s it will now match any amount of white space. Again, see perlretut for more info.