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my $cmd_tmpl = {
  backup => '$bak_cmd $client > $file',
...

# all the names of the vars to be substituted
my @all_my_vars = qw / bak_cmd client file list_cmd ... / ;

sub some_func {
   my $func = 'backup';
...
   seurity_check($cmd);
   system("$cmd");
}

sub eval_cmd {
...
   use strict "vars";
   return $cmd;
}

# $client can be given, or not

...
        system('$backup-cmd');
   }
}