my %v; # this hash holds all the data read

# define the names of the "variables" to read from the file
# *** in the correct order ***
my @varnames = qw/this that theother/;

# define where the length to read is not a default
my %varlengths = ('that' => 2, 'theother' = '$v{this}*2'}

foreach my $var (@varnames){

 # get the length to read
 my $readlength;
 if(! exists($varlengths{$var})
  $readlength = 1; # default
 } else {
  $readlength = eval($varlengths{$var}); # compute it
 }
 
 # read into %v
 if ($readlength == 1){
  # read a scalar
  $v{$var} = unpack TEMPL1, read( $file, $readlength );
 } else {
  # read into an anon. array
  $v{$var} = [ unpack TEMPL1, read( $file, $readlength ) ];
 }

}