How can I use a variable instead of "F28"?

This essentially just goes back to toolic's original reply: make sure that  $sheet is a hash reference and that its content is what you expect; access the contents via  -> arrow notation, forming the hash keys as before with something like  $name = $sheet->{"${_}28"}; etc. (A simple concatenation like  $sheet->{$_ . '28'} would work just as well.)

#!/usr/bin/perl -w
use warnings;
use strict;
use Spreadsheet::Read;
my $name = "";
my $strain = "";
my $initdensity = "";
my $finaldensity = "";
my $avedensity = "";
my $book;
my $sheet;
my %sheet = ();
$book = ReadData ($inbook);
my @x = qw(F G H I J K);
for my $sheet_index (1 .. $sheet_count) {
    $sheet = $book->[$sheet_index] or next;
    foreach (@x) {
       $name = $sheet{'F28'};
       $strain = $sheet{'F29'};
       $initdensity = $sheet{'F30'};
       $finaldensity = $sheet{'F31'};
       $avedensity = $sheet{'F32'};
       print "Found sheet with label: $sheet{label}\n";
       print "COL=$_ $name $strain $initdensity $finaldensity $avedens
+ity\n";
    }
}
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Thanks, Hammer

I'm not at all familiar with Spreadsheet::Read (in fact, I'm not greatly familiar with spreadsheets in general), but the code you show seems close to being workable. Do this:

• Get rid of the whole
      my %sheet = ();
  statement. In the code you show, it seems to be doing nothing more than confusing the heck out of you.
• The  foreach (@x) { ... } loop seems OK for looping over the values of the  @x array, with the  $_ default scalar being "topicalized" to each column-name string in turn.
• Form the cell name as has been discussed and access the cell within the  $sheet hash reference (I'm assuming this is a valid operation) using the  -> arrow operator, e.g.
      $name = $sheet->{"${_}28"};
  or perhaps
      $name = $sheet->{$_ . '28'};
  
• Report results. Don't just say "it doesn't work", that's almost useless. Describe your expectations and report how the code fails to meet your expectations. Report exact warning or error messages.
• Ponder the Basic debugging checklist.

Good luck.