Can anyone explain why a calling code-ref to a "fake" sub causes this recursion?

Because it's just like a normal sub calling itself since the coderef is pointing to the sub that is currently being called.

A way around calling the fake sub would be to see if the coderef was pointing to an actual sub living in a symbol table somewhere

#!/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;
[download]

This works because LOCK_EX (which is what $lock_ex_coderef points to) isn't defined at that point.

Another (albeit more hackish) way to see if a dummy sub is being called is to see if the coderef and the symbol table entry have the same reference value

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;
[download]

This is very icky (it won't pass string any time soon ;-), but somewhat effective as it halts recursion by seeing that it would be calling itself. Also having written the code I learned that $lock_ex_coderef is merely pointing to an entry in the symbol table whereas &Fcntl::LOCK_EX is pointing to a coderef (which is why the $lock_ex_coderef assignment had to go after the &Fcntl::LOCK_EX assignment).
HTH

_________
broquaint

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