while (1) {
   ...
}

##</code><code>##

my @a;
while (my $val = <>) {
   chomp $val;
   push @a, $_ if !grep $_ eq $val, @a;
}

##</code><code>##

my @a;
my %seen;
while (<>) {
   chomp;
   next if $seen{$_}++;
   push @a, $_;
}