$ perl /home/tux/.local/bin/foo.pl [...]
Argument "" isn't numeric in addition (+) at
	/usr/share/perl5/Selenium/CanStartBinary.pm line 111 (#1)
    (W numeric) The indicated string was fed as an argument to an operator
    that expected a numeric value instead.  If you're fortunate the message
    will identify which operator was so unfortunate.
    
Uncaught exception from user code:
	Argument "" isn't numeric in addition (+) at /usr/share/perl5/Selenium/CanStartBinary.pm line 111.
	Selenium::CanStartBinary::_build_binary_mode(Selenium::Firefox=HASH(0x24f1958)) called at (eval 66) line 18
	Selenium::CanStartBinary::binary_mode(Selenium::Firefox=HASH(0x24f1958)) called at /usr/share/perl5/Selenium/CanStartBinary.pm line 53
	Selenium::CanStartBinary::__ANON__(Selenium::Firefox=HASH(0x24f1958), 1) called at (eval 64) line 522
	Selenium::Firefox::new("Selenium::Firefox", "startup_timeout", 10) called at /home/tux/.local/bin/foo.pl line 48

my $driver = Selenium::Firefox->new(
    startup_timeout => $startup_timeout,
);