it uses grep, puts your arguments in a hash, and you can easily set defaults, and it automatically makes your "usage" string, and it automatically rejects invalid arguments. arguments are in "option=value" style to avoid ambiguity

#!/usr/bin/perl -w

# set defaults here
%args = ( width => 750,
          height => 600,
          invert_threshold_factor => 0.25,
          invert => -1,
          pscanhome => "/home/lab/pscan",
          color_a => "darkcyan",
          color_b => "darkred");

$options = '[' . (join '=?] [', sort keys %args) . '=?]';
$usage = "usage: $0 $options picname\n";
$name = pop || die $usage;

foreach (@ARGV) {
   my ($a, $b) = split '=';
   die $usage unless grep {$a eq $_} keys %args;
   $args{$a} = $b;
}
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