sub extractData
{
    my @projects;

    push( @projects, [readOnly("$proj Data.txt"), "A"] );

    unless( $pointerProj eq "NONE" )
    {
        push( @projects, [readOnly("$pointerProj Data.txt"), "B"] );
    }

    for( @projects )
    {
        my @data = @{ $_->[0] };    # WAS: my @data = @{ $_[0] };
        my $key = $_->[1];          # WAS: my $key = $_[1];
        ...
    }
}

sub readOnly
{
    my @contents;
    my $filePath; my $file;
    my $errMsg;

    $filePath = "$path\\" . $_[0];
    $errMsg = "Unable to read $filePath: $!";

    open $file, '<', $filePath or die $errMsg;

    @contents = <$file>;

    close $file;

    return \@contents;              # WAS: return \@contents;
}
[download]

I changed readOnly() to return an array reference. Each project is then an array reference with another array reference (the 'data') at index 0 and a text string (the 'key') at index 1. To get these two items, you need to dereference the outer array reference with $_->[X]. You used $_[X] which is how you get an item from an array, not an array reference.

I tried updating readOnly() to return a reference, and using -> as you indicated, but still no dice - I get the same error message.

I realize its hard to debug without a living program so I appreciate your effort. I cannot share more than I already have.

Previously I had readOnly() returning @contents and could store the result as follows:

my @data = readOnly( "data.txt" );

With readOnly() returning \@contents, how would I then store the result?

my @data = ?

Is this an accurate depiction of what you are attempting?

perl -E '
use strict;
use warnings;
my @projects;
push @projects, [ [ "the", "data" ], "key"];
push @projects, [ [ "more", "data" ], "another key"];
for (@projects) {
  my @data = @{ $_->[0] };
  my $key  = $_->[1];
  say "DATA: @data";
  say "KEY:  $key";
}
'
[download]

OUTPUT:

DATA: the data
KEY:  key
DATA: more data
KEY:  another key
[download]

I suggest you print (or use Data::Dumper/Data::Printer on @contents before it is returned and on $_ in the for loop to see what all is going on.

While I appreciate that a full program would be preferable, I do not see why it should be necessary to have the entire program to educate someone on how to correctly pass an array or array reference through multiple structures.

As I work in a professional environment, I can only share snippets of code and I have to "wash" them to some extent before sharing. I appreciate that this makes assistance a more difficult task.