As an alternative to iterating over @ARGV and opening each file yourself, you could use a combination of <> and $ARGV:

my %total;
my %by_file;
while (<>) {
  chomp;
  $total{$_}++;
  $by_file{$ARGV}{$_}++;
}
[download]

A simple output routine:

my @strings = sort keys %total;
for my $string ( @strings ) {
   print( "\t$string" );
}
print( "\n" );

for my $file ( keys %counts ) {
   print( $file );
   for my $string ( @strings ) {
      print( "\t$by_file{$file}{$string}" );
   }
   print( "\n" );
}
[download]

The file names are passed to the script in same fashion as in my first post.