Why not go the following route?

sub file_name {
    my ($client_number, $MM, $DD, $S) = @_;
    my @varnames = qw($client_number $MM $DD $S);
    for (my $i=0; $i <= scalar @_; $i++) {
        die "$varnames[$i] not defined\n" unless defined($_[$i]);
    }
    "I$client_number$MM$DD$S";
}
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Philosophy can be made out of anything. Or less -- Jerry A. Fodor