look in perldoc perlvar for $/ -- it's the input record separator. So print $sheet->Name, $/; is just a fancier, more generic way to do print $sheet->Name, "\n";

what's wrong with using a temp variable? You can always turn the loop into a map and get an array:

# Loop method from above:
foreach my $sheet ( in $xls->Worksheets ) {
    print $sheet->Name, $/;
}

# one line to get array of names:
my @names = map { $_->Name } in $xls->Worksheets;
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As for the warning, it's because you have my $current_sheet = $sheet->Name, $/; but i think it was intended as: my $current_sheet = $sheet->Name . $/;