qx// (also known as backticks) grab only stdout. If the program you're using outputs everything to stderr, qx// will return an empty string (or empty list) accordingly.

You can use shell redirects to redirect 2 (stderr) to 1 (stdout).
@foo = `bar 2>&1`

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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