open my $in, '<', $infile
    or die "Couldn't open '$infile': $!";
open my $out, '>', $outfile
    or die "Couldn't create '$outfile': $!";

my %seen;
while (<$in>) {
    my $key = $in; # change this to whatever key generation you need
    if (! $seen{ $key }++) {
        print $out $in;
    };
};