The bracket operator around data creates an array reference.

my @num_weights = [36,264,188];
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At this point, @num_weights has a single scalar element, which is itself a reference to another array.

Same as:

my @sub_array = (36,264,188); ##real array
my @num_weights = (\@sub_array); ##real array containing 1 reference
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Btw backslash is the make-reference operator

Use parentheses to declare your arrays and it should work.

This modified code:

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
                say Dumper(@num_weights);
            }
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Prints:

$VAR1 = 1;
$VAR2 = 36;
$VAR3 = 264;
$VAR4 = 188;
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