I would use:

$number =~ s#(.{4})#$1 #g;
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There should be a quick way doing this with unpack(), but the heat just got to me... ;-)

Liz

Update
Hmmm... that puts a space at the end. which may or may not be a problem. And it doesn't check for digits. So I guess this would be better:

$number =~ s#(\d{4})(?=\d)#$1 #g;
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or:

$number = "@{[unpack '(A4)*', $number]}";
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but that is heading towards obfuscation ;-) (thanks tedrek!)