Looks like Spreadsheet::BasicRead doesn't have the ability to select sheet by name. But it does allow you to select each sheet by number, and then query its name... so:

sub sheetNames {
    my $xlsx = shift;
    my %ret;
    my $sheet = $xlsx->getFirstSheet;
    while ($sheet) {
        $ret{$xlsx->currentSheetName} = $xlsx->currentSheetNum;
        $sheet = $xlsx->getNextSheet;
    }
    return %ret;
}

sub selectSheet {
    my ($xlsx,$name) = @_;
    my %available = sheetNames $xlsx;
    $xlsx->setCurrentSheetNum($available{$name});
}
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And then before your csv export code runs, do:

selectSheet $ss2, 'My Great Sheet';
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Error checking should be added of course, currently it'll just silently fail if the sheet you choose doesn't actually exist.