To clarify a little, especially for a novice, i would write thundergnat's line:

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

# 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 mes
+sage for reference
open FILE, '<', $filename or die "Cannot open file '$filename': $!\n";
while(<FILE>){
 ...
}
close FILE;
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Also worth mentioning here to OP is that using Data::Dumper for debugging can be very useful (sort of trivial here, but a good general technique):

use Data::Dumper;
die Dumper \@ARGV;   # see what's really in there ...
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Also note that further down the line you may want to check out any of the GetOpt or other modules that do nice command-line parameter processing for you...