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

##</code><code>##

# 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} ...};