use warnings;

open(F, '<1.txt') || die $!;
chomp(@list1 = <F>);
close F;

open(F, '<2.txt') || die $!;
chomp(@list2 = <F>);
close F;

$list{$_} = 1 for @list1;
delete $list{$_} for @list2;

open(F, '>>3.txt') || die $!;
print F $_.$/ for keys %list;
close F;