And if you fix it to $main::a, it'll be pushing a ref to an array with one element, which element is an array ref, which isn't what you want either.

Actually,that is exactly what he wants and what both you and I have described. :) The element in the array ref is in fact a value and not another array.

$ cat pm1080886.pl 
#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my @hd = ( [] );
my $a = [0.25];
my $b = [0.25, 0.5];
push @hd, $a;
push @hd, $b;

print Dumper(\@hd);

exit;

__END__
$ ./pm1080886.pl 
$VAR1 = [
          [],
          [
            '0.25'
          ],
          [
            '0.25',
            '0.5'
          ]
        ];
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