FAQ 5.6 How can I manipulate fixed-record-length files?

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5.6: How can I manipulate fixed-record-length files?

    The most efficient way is using pack() and unpack(). This is faster than
    using substr() when taking many, many strings. It is slower for just a

    Here is a sample chunk of code to break up and put back together again
    some fixed-format input lines, in this case from the output of a normal,
    Berkeley-style ps:

        # sample input line:
        #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
        my $PS_T = 'A6 A4 A7 A5 A*';
        open my $ps, '-|', 'ps';
        print scalar <$ps>;
        my @fields = qw( pid tt stat time command );
        while (<$ps>) {
            my %process;
            @process = unpack($PS_T, $_);
            for my $field ( @fields ) {
                print "$field: <$process>\n";
            print 'line=', pack($PS_T, @process ), "\n";

    We've used a hash slice in order to easily handle the fields of each
    row. Storing the keys in an array means it's easy to operate on them as
    a group or loop over them with for. It also avoids polluting the program
    with global variables and using symbolic references.


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