Or if you're trying to preserve all the lines starting with the line starting with "2", then:

    my $found_2;
    while (<INFILE>) {
        my $line = $_;
        if ( !$found_2 ) {
            if ( $line ~= m/^2/ ) {
                $found_2 = 1;
            } else {
                substr($line,  99, 1, "9");
                substr($line, 148, 1, "Z");
            }
        }
        print OUTPUT $line;
    }
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