Looking for the best way to exit out of a script if a specific scalar is undefined.

Example:
$foo_path = ('/mnt/foo') if ($^O eq "linux");
$foo_path = ('\\\C:\foo.exe') if (($^O =~ /mswin32|nt/i));

[download]
I want to be able to write something like:
if undef ($foo_path) {
   print "\nFOO was not intended to run on OS:$^0\n"
   exit;
}

[download]
... but using undef in this way doesn't work. I've googled
"undef" but am still a bit confused. What's the best way
to verify if a scalar has been initialized and exit the script if it hasn't?
Thanks.

In reply to Using UNDEF by neo1491

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