Try something like this. You need a hash so that you do not store repeated elements.

#!/usr/bin/perl

use strict;

my %record;

while (my $line = <DATA>) {
    $record{$line}++ if ( $line =~ /\S/ );
}

print for sort keys %record;

__DATA__
a

b
c

d

a
a
c

d
c
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