Presumably you have a loop reading the log file like:

while (my $logline = <LOG>) {
    # parse it
}
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Making a process max out at a certain percentage of CPU is not easy to do (because it's generally not a useful thing to do; if the CPU isn't otherwise used, why not use 100%? and if there are other things running, setting different priorities works out better) especially I think on Windows. It may be sufficient for your purposes just to put in a line like sleep 1 unless $. % 100 which makes perl stop for at least a second every 100 lines.

However, you may be able to get the linux code you posted to work by getting rid of sub Cat entirely and making CPU_used like:

sub CPU_used {
   (map $_->[0]+$_->[1], [times])[0]
}
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But unless I'm totally misunderstanding it, you should be using CPU_max(.2) to get a max of 20% CPU use.

Update: note that calling CPU_max every iteration is counterproductive; I'd figure out how many lines you process per second and make your code:

CPU_start();
while (my $logline = <LOG>) {
    # parse it
    CPU_max(.2) unless $. % $lines_per_second;
}
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(using your lines per second figure, and adjusting if needed for finer granularity.)