The following probably gets close to what you want to do. Note $#{$spec_cps[$j]} is used to get the index of the last item in the array referenced by $spec_cps[$j]. See Re^2: print arrays a(i), b(i,j) c(i,j) if you don't understand what I mean by that.

use strict;
use warnings;

my @energy_bin = (1, 2, 3);
my @spec_cps = ([5, 6], [7, 8], [9, 10]);
my @sigma_cps = ([15, 16], [17, 18], [19, 110]);

for my $j (0 .. $#energy_bin) {
   printf "%.5f", $energy_bin[$j];

   for my $i (0 .. $#{$spec_cps[$j]}) {
      printf " %.5e %.5f", $spec_cps[$j][$i], $sigma_cps[$j][$i];
   }
   
   print "\n";
}
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Prints:

1.00000 5.00000e+000 15.00000 6.00000e+000 16.00000
2.00000 7.00000e+000 17.00000 8.00000e+000 18.00000
3.00000 9.00000e+000 19.00000 1.00000e+001 110.00000
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