To avoid puzzling problems with missing data files, test your input with something along these lines:

#!/usr/bin/perl
if (!defined $ARGV[0]) { # make sure some input is given
   die "No input file provided: $!";
} elsif (!-f $ARGV[0]) { # input is not a file
   die "Input $ARGV[0] not available as a file";
}
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Then you'll know when you've got no data file to work on.

Also suggested is the use of "use warnings" and "use strict" at the beginning of nearly every Perl program.