Just that I'd limit the counter variable to the routines that need it; so it seems that the variable doesn't exist in parts of your class that are outside the scope created by the outermost brackets below.

{
    my $count;  # Number of class instances that exist.

    sub BUILD {
        ++$count;
        #
    }

    sub DEMOLISH {
        --$count;
        #
    }
}
[download]

Be well,
rir