use strict;

sub file_name {
  no strict 'vars';
  no strict 'refs';
  my @arg = qw(client_number MM DD S);
  for (@arg) {
    $$_ = defined($_[0]) ? shift : die "$_ not defined"
  }

  "I$client_number$MM$DD$S"
}

my $out = file_name 1,2,3,4;

warn $out;

##</code><code>##

{ local $" "I@_" }