You may find this File Locking tutorial useful. This is a typical basic method. We use a suck it and see approach. If we can't get a lock on the file we die.

open(MYFILE, ">>$myfile") || die "Can't append to $myfile, Perl says $
+!";
flock(MYFILE, 2) || die "Can't lock $myfile, Perl says $!";
print MYFILE "Hello World!\n";
close MYFILE;
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Do you realise that you have written an infinite loop? Your outer loop:

for ($i=0; defined($i); $i++){
    print "Help! I'm trapped in an infinite loop\n";
}
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will never end as the exit condition defined($i) will always be true. If you really want an infinite loop you could just write:

while (1) {
   print "foo";
}

# or you can write

for (;;) {
   print "foo"
}
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In either case you will need break out of the loop using a last, unless you are in one of those moods and want to hang the system :-)

Cheers

tachyon