Aha, I missed the few key words in your node about calling a module, my apologies. As far as "a lot of extra work", it may be a simple application now, but damned if those things don't grow at the worst possible times. ;-)

As far as X/CLI, the most reasonable algorithm is:

my $have_X = 
    (!$opt_forceCLI and $ENV{DISPLAY} 
    and try_Xconnection($ENV{DISPLAY}) 
    or $opt_forceX;
exec ($have_X && !$opt_useconsole) ? "$app.gui" : "$app.X";
[download]

In other words, check the DISPLAY and try to connect to the X server; if that fails, use the CLI. (Also makes certain to handle the command line options to force X or CLI). Yes, this isn't 100%. Yes, there are more complex ways to do it.

In reality, since you are allowing the user to force one or the other via a command-line, you needn't even try to connect to X. With unusual configurations where DISPLAY is set without X running (or vice-versa), the user simply needs to specify a force option on the command line.