The two subroutines in the code below achieve the same thing in different way. As others have pointed out, the mode +< (or O_RDWR with Fcntl module) is the one you need instead of +>> (the latter is still possible, just harder) for the first sub. The second sub uses Tie::File module that makes file operation is as simple as array operation.

The code (file locking is omitted intentionally): 

#!/usr/bin/perl

use strict;
use warnings;

my($rdwr_file, $tie_file) = @ARGV;

with_rdwr();
with_tiefile();

sub with_rdwr {
    open DAT, "+<$rdwr_file" or die "can't open $rdwr_file: $!\n";
    my $last = (<DAT>)[-1];
    if ($last =~ /(\d+)/) {
        my $new = $1 + 1;
        seek DAT, 0, 2 or die "Can't seek in $rdwr_file: $!\n";
        print DAT $new, "\n";
        print "new value for $rdwr_file: $new\n";
    }
    close DAT;
}

sub with_tiefile {
    use Tie::File;
    my @lines;
    tie @lines, 'Tie::File', $tie_file or die "can't tie $tie_file: $!
+\n";
    my $last = $lines[-1];
    if ($last =~ /(\d+)/) { # sure, testing on $lines[-1] saves a line
        my $new = $1 + 1;
        push @lines, $new;
        print "new value for $tie_file: $new\n";
    }
    untie @lines;

}

[download]

The run: 

$ echo 0 > rdwr
$ echo 0 > tie
$ cat rdwr
0
$ cat tie
0
$ perl prog.pl rdwr tie
new value for rdwr: 1
new value for tie: 1
$ cat rdwr
0
1
$ cat tie
0
1
... and some executions later..
$ cat rdwr
0
1
2
3
4
5
$ cat tie
0
1
2
3
4
5

[download]

Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!

In reply to Re: Filehandles vs Uninitialized Values in pattern match by naikonta
in thread Filehandles vs Uninitialized Values in pattern match by Flubb

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.