Using sprintf to stringify a number, you may want to check its range to avoid negatives or overflow surprises:
my $id = shift;
# disallow ID 0;
$id > 0 && $id < 1e9 or return q();
$id = sprintf '%0*lu', ($id < 1e6 ? 6 : 9), $id;
...
Or, you could let perl stringify the number, and munge it from there.
sub id2path {
my $id = shift;
# allow ID 0; disallow 12.34
$id =~ m/^\d{1,9}$/ or return q();
my $n = length($id) > 6 ? 3 : 2;
$id = sprintf '%0*s', 3 * $n, $id;
$_ = "/$_/" for substr($id, $n, $n);
# or: substr($id, $_, 0, '/') for -2*$n, -$n;
return $id;
}
Interestingly, perl sprintf allows zero-padding of
%s! Is this a documented feature?