By default, backticks only capture STDOUT, not STDERR. The 'uninitialized' message appears on STDERR. To capture both STDOUT and STDERR, use:

my $test_ls = `lxs 2>&1`;
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That will remove the warning message.

I was browsing around CPAN, and I stumbled uopn Test::Cmd. I have never tried it, but it does seem relevant.