I like BrowserUk's solution below, except that I'd probably rewrite it (I mostly didn't like the chained if-elsif's) in a manner similar to (untested:)

#/usr/bin/perl

use strict;
use warnings;
use 5.010;

BEGIN{ @ARGV=map glob, @ARGV }

print "File Number    A    A%    B    B%    Null    Null%";

my $default = '';  # set to something sensible, the empty string seems
+ good.
my @allowed = (qw/stringa stringb/, $default);
my (%count, $filenum);

while(<>) {
    chomp;
    $count{$_ ~~ @allowed ? $_ : $default}++;
    if (eof) {
        $filenum++; 
        say "$filenum       ", join '    ' =>
          map { my $x=$count{$_}; $x, sprintf('%.2f', $x/1000) } @allo
+wed;
        @count{@allowed}=(0) x @allowed;
    }
}

__END__
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I threw in some 5.10-isms in the course of doing so, but it wouldn't be terribly different with pre-5.10 exists.