You want executable code, so you need to use /e. 
s/^([^_]+_).*(\.\w{3})\z/$1 . sprintf("%08d", rand(100000000)) . $2/eg
+;

[download]
or 
s/^([^_]+_).*(?=\.\w{3}\z)/$1 . sprintf("%08d", rand(100000000))/eg;

[download]
or (5.10 required) 
s/^[^_]+_\K.*(?=\.\w{3}\z)/sprintf("%08d", rand(100000000))/eg;

[download]

This way the files will be unique and uniform for my storage needs.

Random numbers aren't unique. They're random.

In reply to Re: Renaming an image file by ikegami
in thread Renaming an image file by Anonymous Monk

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