You cannot. Observe

$ perl -le " printf(qq!%.2e\n!,0.000123); "
1.23e-004

$ perl -le " printf(qq!%.1e\n!,0.000123); "
1.2e-004

$ perl -le " printf(qq!%.3e\n!,0.000123); "
1.230e-004
[download]

You will have to use a different approach.