Tested under WinNT:

use strict;
use Date::Tie;
tie my %dt, 'Date::Tie';
my @a;
for (1..7) 
{
    push @a, join('-', $dt{year}, $dt{month}, $dt{day});
    $dt{day}--;
}
my $re = join('|', @a);

for ( qw( 2003-08-25 2003-08-20 2003-08-27 2003-08-15 ) )
{
    print "$_ is ";
    print "not " unless /$re/; 
    print "in last 6 days\n";
}

# 2003-08-25 is in last 6 days
# 2003-08-20 is in last 6 days
# 2003-08-27 is not in last 6 days
# 2003-08-15 is not in last 6 days
[download]

update: "full version":

use strict;
use Date::Tie;
tie my %dt, 'Date::Tie';
my @a;
for (1..7) {
    push @a, join('-', 
        ( $dt{day} < 10 ? "(?:0)?".(0+$dt{day}) : $dt{day} ),
        ( $dt{month} < 10 ? "(?:0)?".(0+$dt{month}) : $dt{month} ), 
        "(?:".substr($dt{year}, 0, 2).")?".substr($dt{year}, 2, 2) ,
    );
    $dt{day}--;
}
my $re = join('|', @a);
print "RE: $re\n";

for ( qw( 25-08-2003 25-8-03 20-08-03 27-08-2003 15-8-03 ) )
{
    print "$_ is ";
    print "not " unless /$re/; 
    print "in last 6 days\n";
}

# 25-08-2003 is in last 6 days
# 25-8-03 is in last 6 days
# 20-08-03 is in last 6 days
# 27-08-2003 is not in last 6 days
# 15-8-03 is not in last 6 days
[download]