You could use $\ instead, which will trim the two chars at the expense of an 'uninitialized' warning....

$n=3;$s=1;for(1..9){$n*=2;$s=(2-(4-$s**2)**.5)**.5;$\=$n*$s}print
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Infact, if you rearrange things and toss in a little obfu, you can trim a few more chars. It even has the side-effect of making it strict compliant:

$@=3;$;=1;$@*=2,$;=(2-(4-$;**2)**.5)**.5,$\=$@*$;for 1..9;print
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-Blake