Assuming that the column Name is in the first row , you can determine the column containing "FirstName" by

my $header_Row = $wks->Range("A:A")->{'Value'};
my ($FirstNameColumn) = grep  {  $header_Row->[$_ - 1] =~  /firstname/
+i} 1..@header_Row;  

My $currrentRow="18"; Whatever row you are on now
my $FirstName = $wks->Cells($currentRow,$FirstNameColumn)->{'Value'};
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UNTESTED.