The reason @xls is empty is because you did not assign it any values. Typically, this is done by passing a variable reference to GetOptions for it to assign values to the variable (but using a sub might also accomplish this). It's not clear to me yet what you are trying to do. Please update your post with expected values for your variables.

UPDATE:

use strict;
use warnings;
use Getopt::Long();

sub usage {
    my $message = shift(@_);
    if ( defined $message && length $message ) {
        $message .= "\n"
          unless $message =~ /\n$/;
    }

    my $command = $0;
    $command =~ s#^.*/##;

    print STDERR ( $message, "usage: $command -f file(s).xls -p file.c
+sv\n" );

    die("\n");
}

my @xls;
my $csv;

print "This is the number of \@ARGV arguments before the process " . @
+ARGV . "\n";

Getopt::Long::GetOptions(
    'f=s' => \@xls,
    'p=s' => \$csv,
) or usage("Invalid commmand line options.");

usage("The '*.xls' file(s) needs to be specified.")
  if scalar( @xls == 0 );

usage("The '*.csv' file needs to be specified.")
  unless defined $csv;

print "Input given form \@xls: " . @xls . "\n";
print "Input given form \$csv: " . $csv . "\n";

use Data::Dumper;
print Dumper(\@xls);

__END__

Here is my output:

This is the number of @ARGV arguments before the process 6
Input given form @xls: 2
Input given form $csv: test.csv
$VAR1 = [
          'sample.xls',
          'test.xls'
        ];
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