$| = 1; # autoflush required
while (<$fh>) {
   spinner if !($. % 100);
   # process line
}

##</code><code>##

{
    my @spin = qw(- \\ | /);
    my $time = time;
    sub spinner {
        if ($time != time) {
            ++$time;
            my $spin = shift @spin;
            print STDOUT "\r$spin";
            push @spin, $spin;
        }
    }
}