That's correct: you cannot add something to the beginning of a file without rewriting the whole file.

As it seems, the code does not implement one of the required tasks: 'then remove those 3 lines from daily.txt'.

Of course, this can be fixed: 

#!/usr/bin/perl -w

my $daily="daily.txt";
my $weekly="weekly.txt";
my $lines=3;
# alternatively, read the names from the command line:
# my($daily,$weekly,$lines)=@ARGV;

# read the daily file:
open DAILY, $daily or die "open $daily: $!";
my @daily=<DAILY>;
close DAILY;

# read the weekly file:
open WEEKLY, $weekly or die "open $weekly: $!";
my @weekly=<WEEKLY>;
close WEEKLY;

# if the last line of @daily has no \n, this line will
# be merged with the first line of @weekly.  We are
# paranoid and make sure it is there!
$daily[-1]=~/\n$/ or $daily[-1].="\n";
# Well. That's good for Unix, not for Windows. You would
# have to change "\n" to whatever is appropriate.

# move the last $lines lines from the end of @daily to
# the beginning of @weekly. Note:
# splice(@daily, -$lines) removes and returns the last 
# $lines lines from @daily;
# unshift @weekly, @some_lines adds some lines at the
# beginning of @weekly.  Together:
unshift @weekly, splice(@daily, -$lines);

# Write back the weekly file:
open WEEKLY, ">$weekly" or die "open >$weekly: $!";
print WEEKLY @weekly;
close WEEKLY;

# Write back the daily file:
open DAILY, ">$daily" or die "open >$daily: $!";
print DAILY @daily;
close DAILY;

[download]

You might be interested in the splice and unshift manpages for documentation of the main step of this program.

By the way: It is possible to append to the end of a file by opening it with open FILE, ">>$name".

With the truncate function, you can truncate a file (chop off the end leaving the beginning in place). However, I wouldn't recommend that: this function may not be implemented on some systems. Also, it's much simpler to overwrite the file.

In reply to RE: Re: Reading and writing to files by Anonymous Monk
in thread Reading and writing to files by damian

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.