Try Tie::File, here is a piece of its man page:

# This file documents Tie::File version 0.93

tie @array, 'Tie::File', filename or die ...;

$array[13] = 'blah';     # line 13 of the file is now 'blah'
print $array[42];        # display line 42 of the file

$n_recs = @array;        # how many records are in the file?
$#array -= 2;            # chop two records off the end

for (@array) {
  s/PERL/Perl/g;         # Replace PERL with Perl everywhere in the fi
+le
}

# These are just like regular push, pop, unshift, shift, and splice
# Except that they modify the file in the way you would expect

push @array, new recs...;
my $r1 = pop @array;
unshift @array, new recs...;
my $r1 = shift @array;
@old_recs = splice @array, 3, 7, new recs...;

untie @array;            # all finished
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Hope this helps, Valerio

'course, if you're aiming for something a bit less complicated, there's always:
perl -pi -e 's/old text/new text/' filename
...assuming, of course, that that's what you're trying to accomplish (the original question wasn't all that clear on that point)

the_pusher_robot: Basically Iam reading a bunch of lines from a file. One of the line looks like: cdsadmin -t -O :c:\data"; I want to make a copy of this line, then change the the "-t" to "-d" in the copy. I then want to add this new line(copy) back to the file. The file now contains the original lines plus the new one.

Where do you want to add the new (changed) piece of text? Immediately after the line you modified? At the end of the file? Somewhere else?

A solution like the_pusher_robot's can probably be made to work for you. This might do the trick:

perl -pi.bak -e 's/^cdsadmin\s+-t(.*)/${_}cdsadmin -d$1/' yourfile
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I do recommend providing an argument to -i in order to create a backup of the original otherwise a typo might ruin your whole day. ;-)

-sauoq
"My two cents aren't worth a dime.";