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.

In reply to Re: How to control no. of digits on exponent by Anonymous Monk
in thread How to control no. of digits on exponent by mbhuiyan

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