There are serious flaws with this code.

For example, my @species = split "//", $list;. The @species array will contain each letter of $list (a split on the null string is every letter of the original string).

update: Opps... @species = split(//,$list) is not @species = split("//",$list). A cross-eyed booboo on my part. Sorry.

@tree will contain the lines returned from the "backtick" command in the subroutine. The foreach loop is the main program as presented.

my @tree_array;
# my @species = split "//", $list; 
# would:
#   foreach my species ('a','b','c') {} work? I think not?
foreach my $species(@species)
{
     my @tree = edit($species);
     push @tree_array, [@tree] if @tree;
     print @tree if @tree;
}

## do you want to use @tree here?
## if so, then ...you need to use the array
## of the @tree results...

foreach $tree_ref (@tree_array)
{
    print "@$tree_ref\n";
}

sub edit
{
    my ($species) = shift;  #slightly faster when just one arg
    
    my ($pid) = ($species =~ /RANK.*?subspecies.*?PARENT ID.*?(\d+)/);
    ### or ###
    ## look for "PARENT ID", then anything until there are
    ## one or more digits
    my ($pid) = ($species =~ /PARENT ID.*?(\d+)/); 
    
    if (defined $pid)
    {
        return `getz "[taxonomy:$pid]" -e`;
    }
    return undef;
}
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Feel free to modify the above code, show a runnable example of your problem.