If a taint check for relative paths were to be added, it would throw an exception rather than setting $! to file not found.

Note that this taint check would be a departure from the current model which allows the current directory to be used.

$ cat script.pl
#!/usr/bin/perl
print "unsafe?\n";

$ perl -T -e'$ENV{PATH}=""; system "script.pl";'
unsafe?
[download]

$ cat Mod.pm
print "unsafe?\n";
1;

$ perl -T -e'@INC = "."; require Mod;'
unsafe?
[download]

Sounds like a pretty good idea to me, though. It would break stuff, but nothing that can't be fixed.