if ( $seen{ $elem } ) {
    next; # skip to the next loop iteration
}
else {
    $seen{ $elem } += 1;
}

##</code><code>##

use strict;
use warnings;

open my $IN, '<', $ARGV[0]  or die("Can't read $ARGV[0]: $!");
open my $OUT, '<', $ARGV[1] or die("Can't write $ARGV[1]: $!");

my %seen;
while (<$IN>) {
   next if $seen{$_}++; # check/mark line as seen
   print $OUT $_;
}

close $IN; close $OUT;

##</code><code>##

while (<$IN>) { $seen{$_}++ || print $OUT $_ }