You can, if you _really_ must encapsulate the setting environment into the 'backticks'.

my $result = `set LIBPATH=/opt/freeware/lib:/opt/freeware/64/lib; ssh 
+$host stuff`;
[download]

It's probably better to configure %ENV though. Any may be better yet to fix your library paths though - ssh is something that'd probably benefit from being 'properly installed' into the environment. Maybe even by replacing your ssh binary (in your path) with a wrapper script, in /usr/bin or similar. It may be worth doing a (non packaged) install of ssh, so you can install your own libs in the right places.