in reply to "Nonliteral literal" is just an expression.

This calls for recursion. 
#!/usr/bin/perl
use warnings;
use strict;

use List::Util qw{ any };

sub flatten {
    my ($struct) = @_;
    if (ref [] eq ref $struct) {
        return map flatten($_), @$struct
    } elsif (ref \\"" eq ref $struct) {
        return $$struct
    } else {
        return \$struct
    }
}

use Test::More tests => 2;

my $ar = [ [ 'HEAD_goes_here', 'BODY_here' ],
           [ 'FOOT', 'FOOT', 'FOOT', 'FOOT' ] ];

my @flat = @$ar;
while ( any { ref [] eq ref $_ } @flat ) {
    @flat = map { ref []   eq ref $_ ? \(@$_) : \$_ } @flat;
    @flat = map { ref \\"" eq ref $_ ? $$_    : $_  } @flat;
}

is_deeply \@flat,
    [map \$_, qw[ HEAD_goes_here BODY_here FOOT FOOT FOOT FOOT ] ];

is_deeply [flatten($ar)],
    \@flat;

[download]

What's wrong with the "less helpful" template? Both approaches return the same result.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]

Replies are listed 'Best First'.
Re^2: "Nonliteral literal" is just an expression.
by rir (Vicar) on Sep 24, 2019 at 21:30 UTC
    What's wrong with the "less helpful" template?

    That is the interesting part. And it surprises me that I have never been caught by this in my years of occasional programming. I am still chewing on that aspect.

    Both approaches return the same result.

    Not quite, your tests are flawed. I wanted references to the same memory objects ("thingie" is out of style?) so running @flat through both your and my code is suspect on its face. When I comment out the while loop I don't get a happy result.

    On a side note: In the "not-thinking" part I started on a recursive solution; when I hit some snags and applied some thought map seemed a clear win. I should study the methods to go from one to the other: iter. vs. recurs.

    Using a shorter array: 

    my $ar = [ [ 'a', ], [ 'b, 'c', ], ];
my $flat = flat( $ar );
print Dumper $ar, $flat;

    [download]
    should yield: 
    $VAR1 = [
          [
            'a'
          ],
          [
            'b',
            'c'
          ]
        ];
$VAR2 = [
          \$VAR1->[0][0],
          \$VAR1->[1][0],
          \$VAR1->[1][1]
        ];

    [download]
[reply]
[d/l]
[select]
      Ah, now I understand! A small change in the flatten subroutine should fix that. 
      sub flatten {
    my ($struct) = @_;
    if (ref [] eq ref $struct) {
        return map flatten($_), @$struct
    } else {
        return \$_[0]  # <- We need to use the alias, not a copy.
    }
}

      [download]
      map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
[reply]
[d/l]
[select]

In Section Meditations