Because you're still vulnerable to the race condition, it's just very timing dependent. Here's how the timeline falls out. At first, the file doesn't exist. 
unless ( -l "foo" ) { # we get here fine
    #*now* the link is created
    open( FH, ">>foo" ) or die "foo: $!";
    #and now it's removed again
}
die "Link attack detected" if ( -l "foo" );
#too late, the link is gone already, so we didn't die.

[download]
Now, it's really, really hard to get the timing to work perfectly for that, but that doesn't mean that it won't happen sometimes. So if you start a program creating and removing the link very quickly, and at the same time run the other program again and again, sooner or later you're going to get unlucky.

In reply to Re^3: opening files: link checking and race conditions by Eimi Metamorphoumai
in thread opening files: link checking and race conditions by danderson

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