Generate the random number by sticking together N random digits to get. For example, if you want a random number between 1e30 and 9.99e39:

my $num = join '', map { int rand 10 } (0..(30 + int rand 10) );
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Update actually, you don't even need the rand at the end; to go from 0 to 9.99e39:

my $num = join '', map { int rand 10 } (0..39);
[download]

as numbers of 38 digits or less will have zeroes prepended to them.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important