#!/usr/bin/perl -w

use Fcntl qw(LOCK_EX);

my $lock_ex_coderef= \&Fcntl::LOCK_EX;;
# instrument LOCK_EX
*Fcntl::LOCK_EX = sub { 
    print "LOCK_EX called\n";
    return &$lock_ex_coderef if defined &$lock_ex_coderef;
};

# call LOCK_EX
$_ = LOCK_EX;