I had a feeling the "eval" would be needed,

If your argument was Perl code, sure.

$ perl -e'
    my $host = "localhost";
    eval $ARGV[0];
' 'system("echo", $host)'
localhost
[download]

But your argument is a shell command. That means you'll need to parse the argument as if it was a shell command an interpolate the appropriate variables.

It would be easier to use a template.

$ perl -MTemplate -e'
    Template->new()->process(
        \$ARGV[0],
        { host => "localhost" },
        \my $cmd
    );
    system($cmd)
' 'echo [% host %]'
localhost
[download]