first of all, you have to mark the period as a special character:

/(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3})/
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because else you mean "any one to three digits followed by any sign", which might not be what you have in mind. but now to your questions:

1.)How to ensure each octet doesn't exceed 255 (e.g. 300.1.2.2 is not a valid IP address)
i should rewrite the regex and the controlling if:

my $ip;
if ($line =~ /(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})/ &&
    $1 <= 255 && $2 <= 255 && $3 <= 255 && $4 <= 255) {
   $ip = "$1.$2.$3.$4";
}
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2) How to distinguish subnet masks from IP address.
the answer is quite simple now, since we already have all four octets:

my ($ip, $subnet);
if ($line =~ /(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})/ &&
    $1 <= 255 && $2 <= 255 && $3 <= 255 && $4 <= 255) {
   $ip = "$1.$2.$3.$4";
   $subnet = 1 if $1 == 255;
}
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