use Fcntl qw(LOCK_EX);

# define here so the &Fcntl::LOCK_EX can see it
my $lock_ex_coderef;

# instrument LOCK_EX
*Fcntl::LOCK_EX = sub { 
    print "LOCK_EX called\n";
    return &$lock_ex_coderef
		unless $lock_ex_coderef eq *{$Fcntl::{LOCK_EX}}{CODE}; 
};

$lock_ex_coderef = \&Fcntl::LOCK_EX;

# call LOCK_EX
$_ = LOCK_EX;