in reply to Command Line Arg contains Variable and more

You could also create environment variables that the shell will be happy to interpolate for you. 

$ perl -e'$ENV{host}="localhost"; system $ARGV[0]' 'echo $host'
localhost

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The above is basically the same as 
$ host=localhost sh -c 'echo $host'
localhost

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In Section Seekers of Perl Wisdom