use Fcntl qw(:DEFAULT :flock);

open (MYFILE1, "<", "$file")
or sysopen (MYFILE, "$file", O_RDONLY)
or die "$!";

flock (MYFILE1, LOCK_SH)
or die "$!";

chomp (@input=<MYFILE>);
close MYFILE1;

# pretending you then did your work on the input, split at the ":"
# and stuck the results back into @output...

open (MYFILE2, ">", "$file")
or sysopen (MYFILE, "$file", O_WRONLY)
or die "$!";

flock (MYFILE2, LOCK_EX)
or die "$!";

foreach $item(@output){
print MYFILE2 "$item"."\n";}

close MYFILE2;
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doing the double open-sysopen technique helps avoid races for opening the file and having that content altered in those few miliseconds.

it's rare, if you have a small site, that two users would access the file at the same time - but i wouldn't take the risk. Besides, w/CGI work - if someone is ~intentionally~ messing w/your system -> you want to ensure that you're opening the right data and not something that was swapped in by a nefarious user. (This is the reason for using 2 separate filehandles too -> MYFILE1 and MYFILE2).

LOCK_EX (exclusive locks) are needed, when writing to files - otherwise, LOCK_SH (shared locks) are all that's needed, i believe, when reading.

also: i'm not sure, if i'd actually use DIE like this in a cgi system, if the user is going to see the output. You probably want to 'die' gracefully -> using WARN, &subroutine, or some other action instead.

for more info - see Camel -> pgs 419-422, 571-573, 712, 714-715, 808-810