do i need to use a counter, or say "if $foo ne $previousfoo", or is there any easier way?

You don't necessarily need to do that - you are using perl after all ...

open(my $fh, "somefile.txt") or die("ack - $!");
my @lines = <$fh>; # let's hope this isn't a large file ...
for (0 .. $#lines) {
    print "a change - $lines[$_]"
        if $lines[$_] ne $lines[$_ - 1];
}
[download]

However it's probably easier just to keep your state in a variable as it's straight forward and doesn't require the slurping of the whole file.
HTH

_________
broquaint