$Sheet->Range("H$lastrow4")->{Formula} = '=sum("H17:H.$Lastrow3")';
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Try this instead:

$Sheet->Range("H$lastrow4")->{Formula} = qq{=sum("H17:H.$Lastrow3")};
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(Look in perlop for qq.) The problem you had is that you were trying to interpolate a variable ($Lastrow3) into a single quoted string (which does not do variable interpolation).