Disregarding a few other errors in that script (like, why did you comment out use warnings;?), I would suspect that the date format in your input file probably isn't as consistent as you think it is. Have you made a run where all you do is print the value of $Date to verify it will always match what you think it does?

On a tangent, if you have control over the files, I suggest you use a more standard date format, such as YYYY-MM-DD, that is easily produced and parsed. You'll save yourself time and headaches down the road.

You are testing for the date matching on every line. This is not what you want. I think what I've put together below gets you closer.

#!/usr/local/bin/perl
use warnings;
use strict;
use Fcntl ();

my @dateParts = localtime ();
my ($day, $month, $year) = @dateParts[3 .. 5];
$year  += 1900;
$month += 1;
my $already_there = 0;

open my $mon, '<', "C:\\temp\\MONTHLY.CSV" or die "Cannot read file $!
+\n";

while ( defined ( my $line = <$mon> ) ) {
    my ( $date ) = split ',', $line;

    if ( $date eq "$day $month $year" ) {
        print "\ file has already been updated today";
        $already_there++;
        last;
    }
}
close $mon;

if ( ! $already_there ) {
    open my $day, "C:\\temp\\DAILY.CSV" or die "Cannot read file $!\n"
+;
    flock $day, Fcntl::LOCK_SH;
    open $mon, '>>', "C:\\temp\\MONTHLY.CSV" or die "Cannot write to f
+ile $!\n";
    flock $mon, Fcntl::LOCK_EX;
    <$day>; # get past header
    while ( <$day> ) {
        print { $mon } $_;
    }
    close $day;
    close $mon or die "Could not complete writing to monthly file: $!\
+n";
}
exit $already_there;
[download]

> But if todays Date is not in column A then copy the data and append to the monthly file.

Your script seems to check every single line in elsif if it doesn't have todays date, and of course there are lines which are older, hence you append.

You'll need to check all lines (or better only the last one) first ALONE in one loop and AFTER this loop (if you haven't exited yet) you can append your data.

Well, whats all this mktime/strftime stuff, why aren't you using that? Why did you turn off the free help?

Here is how I'd write it (minimize nesting loops)

#!/usr/bin/perl --
## 
## 2014-05-29-18:05:27
## 
## perltidy -olq  -csc -csci=10 -cscl="sub : BEGIN END if " -otr -opr 
+-ce -nibc -i=4 -pt=0   "-nsak=*"
#!/usr/bin/perl --
use strict; use warnings;
use POSIX qw/ strftime /;
use Path::Tiny qw/ path /;

#~ Main( @ARGV );
Main( 'C:/TEMP/DAILY.csv', 'C:/TEMP/MONTHLY.csv' );
exit( 0 );

sub Main {
    my( $infile, $outfile ) = @_;
    if( not has_today( $outfile ) ) {
        append_today( $infile, $outfile );
        ...    ## make noise on STDOUT
    } else {
        ...    ## make noise on STDOUT
    }
} ## end sub Main

sub has_today {
    my( $infile ) = @_;
    my $today = strftime( '%Y %m %d', localtime );
    my $fh = path( $infile )->open_raw;
    ...        ## read last 8k, check for $today
}

## assumes infile will fit in memory, if it don't, write more code :)
sub append_today {
    my( $infile, $outfile ) = @_;
    path( $outfile )->append_raw( path( $infile )->slurp_raw );
}
[download]

when i try this i get Unimplemented at C:\testscripts\dir.pl line 31.

When you append, append to temp file and update the last write date. Then mv your temp file to be the new version of the file you're updating.