while (<DATA>) {
    print if /^192.168.[1-9]\d?.([1-9]\d?\d?)$/ && $1 < 256;
}
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1-99 is either one or two digits, with the first digit always being 1 or greater. Then you have to check for a valid 4th pair, which means 1-3 digits with the first being at least 1 and the whole thing being less than 256.