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push @$list, $$items[$i];
comb_integral($items, $group - 1, $list, $i + 1);
pop @$list;

comb_integral($items, $group - 1, [@$list, $$items[$i]], $i + 1);

#!/usr/bin/perl
use strict;
...
    return @returns;
  }
}

push @returns, my @combs = comb_integral($items, $group - 1, $i + 1);
unshift @$_, $$items[$i] for @combs;

Benchmark: running SparkyG, demerphq, integral2, intgrl2_f, intgrl_np,
+ intgrl_pp, iterative, each for at least 1 CPU seconds...
   SparkyG:  1 wallclock secs ( 1.06 usr +  0.01 sys =  1.07 CPU) @ 15
+70.09/s (n=1680)
...
intgrl_pp 3199/s    104%      40%       10%        3%        --       
+-7%      -25%
integral2 3429/s    118%      50%       18%       10%        7%       
+ --      -20%
iterative 4266/s    172%      87%       47%       37%       33%       
+24%        --