Here's a version with better spacing and some bracketing:

@_=(115,-17,6);
print map { chr($_[$.=$_-$_] * $_ ** $.++ + $_[$.] * $_ ** $.++ + $_[$
+.] * $_ ** $.) } $. - $. .. $#_
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The next step, with some constants precalculated:

@_=(115,-17,6);
print map { chr($_[0] * $_ ** 0 + $_[1] * $_ ** 1 + $_[2] * $_ ** 2) }
+ 0 .. 2
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And finally with all constants added in:

@_=(115,-17,6);
print map { chr(115 + -17 * $_ + 6 * $_ ** 2) } 0 .. 2
[download]

First iteration of map: 115*0^0 + -17*0^1 + 6*0^2 = 115*1 + 0 + 0 = 115 = s.
Second iteration: 115*1^0 + -17*1^1 + 6*1^2 = 115 + -17 + 6 = 104 = h.
Third iteration: 115*2^0 + -17*2^1 + 6*2^2 = 115 + -34 + 24 = 105 = i.