Looks like the perfect place for map.

my @b = map {sprintf("%.2f",$_)} @a;
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Oh, you said obfuscated.... hows this for a first step down obfuscated lane:

my @b = split' ',sprintf" %.2f"x@a,@a;
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one more step, and I'm done:

s;;sprintf" %.2f"x@a,@a;e;my@b=split;
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-Blake