In your solution:

• There's a potential for an infinite loop.
• You're running one shell and one shell command more than you need.
• You're vulnerable to two injection attacks. (Only one fixed)
• You're not passing parent Perl's arguments to the child Perl. (Not fixed)
• You're not passing parent Perl's switches to the child Perl. (Not fixed)

Fix:

#!/usr/bin/perl -w
 
if (!defined($ENV{foo})) {
   if (@ARGV > 0 && $ARGV[0] eq 'nested') {
      die("Unable to find value for env var FOO\n");
   }

   exec 'csh', '-c', "source env.csh ; perl $0 nested";
   die("Unable to execute csh: $!\n");
}

printf "%s = %s\n",$_,$ENV{$_} foreach keys %ENV;
[download]

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