I'm not quite sure what you want here. The numeric sort and your question implies that you want the max value? If that is what you want:

#!/usr/bin/perl
use strict;
use warnings;

use List::Util qw(max); #CORE nothing to install

# changed the order to make max value
# in the middle of @numbers

my @numbers = (15,5,7,3,90,9,1,20,13,9,8,
    15,16,2,6,12);
    
print "max is =", max(@numbers), "\n";

__END__
max is =90
[download]

Update: I thought that I should point out that you can re-assign the sorted array back to the same variable....

#!/usr/bin/perl
use strict;
use warnings;

# changed the order to make max value
# in the middle of @numbers

my @numbers = (15,5,7,3,90,9,1,20,13,9,8,
    15,16,2,6,12);
    
@numbers = sort {$a <=> $b} @numbers;

print "max = $numbers[-1]\n";
print "next to max = $numbers[-2]\n";

__END__
max = 90
next to max = 20
[download]

And yes, if 90 appeared twice, this changes things as both of these would be the same.