I suspect that it's due to the fact that $output doesn't contain what you think it does, and the return ($1) is uninitialized.

Better to check for this condition (untested):

sub getToken {
    my $output = `curl -k -s -XPOST (URL) -dgrant_type=password -dclie
+nt_id=(client ID) -dclient_secret=(client secret) -dusername=(usernam
+e) -dpassword=(password)`;   
    if ($output =~ /"access_token":"(.*)", "ex/g){
        return $1;
    }
    else {
        die "output not as expected...\n";
    }    
}
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