Reading this node Arrays: A Tutorial/Reference will help you better learning.

Here is the code I did some changes in your code try and let me know.

use strict;
use warnings;
use Data::Dumper;


my @num_weights = (36,264,188);
my @leftovers = (1,2,3,4);
my @hold_generations;
 my $i = 0;
        
        for(my $p = 0; $p < 1;$p++) #Go through all leftovers
            {
                my $pick = $leftovers[$p];
                unshift @num_weights, $pick; #add item
                    print Dumper(\@num_weights);
                push @hold_generations, @num_weights; #store new array
                print Dumper(\@hold_generations);
                splice @num_weights, 1, 1; #undo adding item
                print Dumper(\@num_weights);
                
                 
            }
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Note:

I do not have features module so I just used print()