In general, I like to approach problems like this in a slightly different way:   I will read “the original file,” one record at a time, in a loop, and decide which records from that file I want to keep.   I can also change the content of each record in any way that I please.   I will then write those records, one at a time, to “the next generation of ‘the original file.’”   So, when the program is finished, I am left with two files:   “before,” and “after.”     I can then compare the two ... the diff command (in Unix/Linux) comes in very handy here.   If I like what I see, I can (separately) throw-away or archive the original file and keep the new one with just a few rename-commands in the shell.   And, if I don’t like what I see, nothing has been lost or harmed.   It is, in other words, a non-destructive process that works for datasets of any size.

This approach works equally well with files of unlimited length, because it’s just being processed line-by-line no matter how long it is.

So, your program might look something like this ... (caution:   extemporaneous coding)

use strict;
use warnings;

# Open the input file, create the output file.

open(INFILE, "</temp/PERL-Samples/myfile.txt") || die "File not found"
+;
open(OUTFILE, ">/temp/PERL-Samples/myfile_out.txt");

while (<INFILE>) {

#Looks like the actual logic here is actually just a filter:
#1. Finds any instance of || and changes it to | |
#2. Finds any instance of | || and changes it to | | |
#3. Finds any instance of || | and changes it to | | |
#4. Finds any instance of | | at the end of a line and 
#    changes it to ||

   $_ =~ s/\|\|/\| \|/g;
   $_ =~ s/\| \|\|/\| \|/g;
   $_ =~ s/\|\| \|/\| \|/g;
   $_ =~ s/\| \|$/\|\|/g;

   print OUTFILE $_;
}

close INFILE;
close OUTFILE;
[download]