use strict; use warnings;

my $last = '';
while(<>)
{
   my @columns = split; 
   next if $columns[0] eq $last;
   $last = $columns[0];
   print;
}

##</code><code>##

use strict; use warnings;
my %seen;
while (<>)
{
   my @columns = split;
   next if exists $seen{$columns[0]};
   $seen{$columns[0]} = 1;
   print;
}