use 5.20.0;
use warnings;

use Spreadsheet::Read;

my $sheet  = 1;
my $column = 2;
my @column = @{ReadData ("file.xls")->[$sheet]{cell}[$column]};
while (@column and !defined $column[-1]) {
    pop @column;
    }
say "Column ${column}'s last defined row = ", $#column;
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Or if you prefer the reference

my $colref = ReadData ("file.xls")->[$sheet]{cell}[$column];
pop @$colref while $#$colref > 1 && !defined $colref->[-1];
say "Column ${column}'s last defined row = ", $#$colref;
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If all columns have equal length, you can skip the mumble-jumble and just use

say "Column ${column}'s last defined row = ",
    ReadData ("file.xls")->[$sheet]{maxrow};
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