use strict;

sub file_name {
  my ($client_number, $MM, $DD, $S);

  my @word = qw($client_number, $MM, $DD, $S);


  no strict 'refs';


  !defined($_[$_]) and die "$word[$_] not defined"
  or ${$word[$_]} = $_[$_] for (0..$#_);

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

file_name 1,2,3,4;