use strict;
use warnings;


my @log = (
'May 18 12:01:01 bar: eht1  BS',
'May 19 13:58:01 foo: eth0: link down',
'May 19 13:58:11 foo: eth0: link up, 100Mbps, bar, lpa quux'
);

foreach my $line (@log)
{
   next unless (index($line, ": link")>=0); # skips BS
                                            # this is very fast
                                            # even running twice for
                                            # link up or link down is fast
   
   print "$line\n";  # focus on link mentioning lines here
}
#May 19 13:58:01 foo: eth0: link down
#May 19 13:58:11 foo: eth0: link up, 100Mbps, bar, lpa quux