I would combine the already suggested named capture groups with interpolated regexes.

my $month = qr{(?<month>\w+)};
# ...etc
my $re_dn = qr{^$month (\d+) (\d{2}:\d{2}:\d{2}) .*: eth(\d): link dow
+n};
my $re_dn = qr{^$month (\d+) (\d{2}:\d{2}:\d{2}) .*: eth(\d): link up,
+ (\d+)Mbps, ([^,]+), lpa (\w+)};
[download]

also consider using the /x modifier for better readability.

if you are concerned about DRY and error avoidance, you can also use a pattern hash and automatically wrap the names later

# sketch, totally untested

# define groups
my %p = (
   month => '\w+',
   ...
);
 
# wrap named groups later
while ( my ($k,$v) = each %p ) {
   $v = qr{(?<$k>)$v};
}

# compose patterns
my $re_dn = qr{^$p{month} $p{day} ...};
[download]

on a side note: you can also chain elsif to avoid always writing next in every if block