I see two possible issues with this but I don't really know what your intent is....

First, the single quotes around $arg probably shouldn't be there. In the shell command they are used to distinguish one argument from the next, which is unnecessary with the list form of system

Second, where are $user $host and $display coming from? You might want to check the value of $location to be sure its what you expect..... perl variables and enviornment variables are different beasties.

In short, give this a shot:

my $command  = "doorway";
my $location = "$user\@$host.$display";
print "Location = $location\n";
my $arg      = "MSG Hello user";
my $ret      = system($command, $location, $arg,);
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-Blake