my $prefix = '/yourapp/index';
my $userid = ...;
my $idx = substr $userid, 0, 3;
my $limitReach = 1;
{
    opendir DIR, "$prefix/$idx/$userid/" or die $!;
    my $count = readdir DIR;
    last if $count >= LIMIT;
    rename "$prefix/$idx/$userid/$count", "$prefix/$idx/$userid/" . $count + 1
        or redo;
    $limitReached = 0;
}

## use $limitReached to decide further action