You only have to remember the last unique-line, a shot from the hip (untested!):

open(FILE, "<", name.ext") or die("couldn't open file: $!");
open(OUT, ">", new.ext") or  die("couldn't open file for writing: $!")
+;
my $cmp = <FILE>;  # the first line
print OUT $cmp;        # is always unique
while(<FILE>) {
    if ($_ ne $cmp) {    # current line unlike last one?
        print OUT $_; # print it out
        $cmp = $_;   # current line new reference
    }                         # otherwise ignore
}
[download]

regards,
tomte

Hlade's Law:

If you have a difficult task, give it to a lazy person --
they will find an easier way to do it.