open(INFH, "<filename.txt");
my @data=<INFH>;
close(INFH);
open(OUTFH, ">outfile.txt");
my %hashdata = ();
foreach my $thisline (@data)
{ $hashdata{$thisline} = 1; }
foreach my $thisline (sort(keys(%hashdata)))
{ print(OUTFH, $thisline) }
close(OUTFH);