my $cmd_tmpl = {
  backup => '$bak_cmd $client > $file',
  list   => '$list_cmd -le $client',
  ...
}

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

##</code><code>##

sub some_func {
   my $func = 'backup';
   my $client = 'mars';
   my $cmd = eval_cmd($func); # here magic happens!

   seurity_check($cmd);
   system("$cmd");
}

##</code><code>##


sub eval_cmd {

   my $key = shift;

   die "no such command: $key!" unless exists $cmd_template{$key};

   my $cmd = $cmd_template{$key};
   no strict "vars";
   for my $var (@all_my_vars) {
      $cmd =~ s/\$var/$$var/g if defined $$var;
   }
   use strict "vars";
   return $cmd;
}

##</code><code>##

# $client can be given, or not

if ($client) {
   some_func();
} else {
   foreach $client (@{get_all_running_clients()}) {
        my $backup-cmd = eval_cmd('backup');
        system('$backup-cmd');
   }
}