I want to be able to ensure a set of variables will all have values before assigning them to @_

But that's not what you're doing here. You're checking that the values in @_ are defined before assigning them to your local variables using a horribly complex statement.

Do you have to know the variable name? Wouldn't the parameter number be enough to track down any problems? I'd probably wrtie it something like this:

use strict;

sub file_name {
  die "incorrect number of parameters\n"
    unless @_ == 4;

  for each (0 .. $#_) {
    die "invalid parameter [$_]\n"
      unless defined $_[$_];

  $" = '';

  "I@_"'
}
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Oh, and the commas in your qw are probably a bug too.

--
<http://www.dave.org.uk>

"Perl makes the fun jobs fun
and the boring jobs bearable" - me