It may be that you want something like:

use strict;
use warnings;

my %dates;

while (<DATA>) {
    next unless /(.)(..)(..)(.{4})/;

    my $date = "$4$3$2";

    if (exists $dates{$date}) {
        print "<TD class=table_font>$date - Hash exists</TD>\n";
        # play with hash
    } else {
        print "<TD class=table_font>$date - Hash doesn't exist</TD>\n"
+;
        # create the hash
        $dates{$date} = undef;
        # play with hash
    }
}

__DATA__
x11112007
y11122007
p11112007
x11132007
[download]

Prints:

<TD class=table_font>20071111 - Hash doesn't exist</TD>
<TD class=table_font>20071211 - Hash doesn't exist</TD>
<TD class=table_font>20071111 - Hash exists</TD>
<TD class=table_font>20071311 - Hash doesn't exist</TD>
[download]

Note that it helps a lot to understand what you are trying to do if you provide a little data and tell us what you expect to see and what you actually see.