You can do it in one shot, so you might as well.

That makes your second snippet

my $prev;

while (<>) {
    next if ($_ eq $prev);
    print $prev = $_;
}
[download]

:^)

Wait, we can shorten that..

my $prev;

while (<>) {
    print $prev = $_ unless $_ eq $prev;
}
[download]

Hmm..

my $prev;
$_ ne $prev and print $prev = $_ while <>;
[download]

Err.. sorry, got carried away for a second.. Perl is just too seductive. Sigh. :-)

Makeshifts last the longest.