open my $fh, shift or die "Cannot open file: $!\n";

##</code><code>##

# do all the assignments from ARGV here
my $filename = shift @ARGV or die "A filename parameter is required";

# explicitly open for read; and make sure filename is in the error message for reference
open FILE, '<', $filename or die "Cannot open file '$filename': $!\n";
while(<FILE>){
 ...
}
close FILE;

##</code><code>##

use Data::Dumper;
die Dumper \@ARGV;   # see what's really in there ...