Hello myfamilygeneral, and welcome to the Monastery!

The problem is not with $dirtree_frame->destroy, but with the immediately preceding term \&show_cwd(). Consider the following:

#! perl
use strict;
use warnings;

sub foo
{
    print "in foo()\n";
}

sub bar
{
    \&foo();
    print "in bar()\n";
}

bar();
[download]

Output:

Useless use of reference constructor in void context at test.pl line 1
+2.
in foo()
in bar()
[download]

But remove the backslash (which does nothing useful anyway) from before &foo(), and the warning goes away.

Update: The & sigil probably does nothing useful either. See What's the difference between calling a function as &foo and foo()?

HTH,