use warnings;
use strict;
@ARGV or die "You need to supply a file name.\n";
open my $fh, '<', shift or die "$!\n";
my @lines = <$fh>;
my %unique;
@unique{@lines} = (1) x @lines;

# unique lines
#print keys %unique;

# unique sorted lines
#print sort keys %unique;

# unique lines in order seen in original file
do { print if delete $unique{$_} } for @lines;