It sounds like this might have been an XY Problem: "How do I count the occurrences of each character in a string/file/etc?" (Caution: The following solution only works for byte characters (i.e., 1 byte == 1 character), not Unicode characters.)

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'A man, a plan, a canal: Panama!';
 ;;
 my %freq;
 ++$freq{ substr $s, $_, 1 } for 0 .. length($s) - 1;
 ;;
 printf qq{'$_' (0x%02x) == $freq{$_} (%6.3f%%) \n},
   ord, $freq{$_} / length($s) * 100
   for sort { ord($a) <=> ord($b) } keys %freq;
"
' ' (0x20) == 6 (19.355%)
'!' (0x21) == 1 ( 3.226%)
',' (0x2c) == 2 ( 6.452%)
':' (0x3a) == 1 ( 3.226%)
'A' (0x41) == 1 ( 3.226%)
'P' (0x50) == 1 ( 3.226%)
'a' (0x61) == 9 (29.032%)
'c' (0x63) == 1 ( 3.226%)
'l' (0x6c) == 2 ( 6.452%)
'm' (0x6d) == 2 ( 6.452%)
'n' (0x6e) == 4 (12.903%)
'p' (0x70) == 1 ( 3.226%)
[download]

i want you to know... i wrote out 1853 lines to achieve what you were able to do in 7 lines lol. and not only that, but on a larger 256 MB file, your code is 3 secs faster(39 secs) as opposed to mine (42 secs). tho i am not sure how HxD is able to do it, but it gets these same statistics on a 256 MB file in 3-4 secs flat.

But, i am going to study this code because it would have saved me hours if i knew exactly how do it this way in the first place. and thanks for sharing :)

my $file = read_file($ARGV[0], { binmode => ':raw' });
my $x00 = $file =~ tr/\x00//;
my $size = (stat($ARGV[0]))[7];
my $dec = $x00/$size;
my $pc = ($dec*100);
my $percentage00 = sprintf("%.2f", $pc);

.....
same code, but tr/\x01// thru tr/\xfe//
.....

my $xff = $file =~ tr/\xff//;
my $dec = $xff/$size;
my $pc = ($dec*100);
my $percentageff = sprintf("%.2f", $pc);

close($file);

my $sum =   $percentage01+$percentage02+$percentage03+$percentage04+$p
+ercentage05+$percentage06+
            $percentage07+$percentage08+$percentage09+$percentage0a+$p
+ercentage0b+$percentage0c+
            $percentage0d+$percentage0e+$percentage0f+$percentage10+$p
+ercentage11+$percentage12+
            $percentage13+$percentage14+$percentage15+$percentage16+$p
+ercentage17+$percentage18+
            $percentage19+$percentage1a+$percentage1b+$percentage1c+$p
+ercentage1d+$percentage1e+
            $percentage1f+$percentage20+$percentage21+$percentage22+$p
+ercentage23+$percentage24+
            $percentage25+$percentage26+$percentage27+$percentage28+$p
+ercentage29+$percentage2a+
            $percentage2b+$percentage2c+$percentage2d+$percentage2e+$p
+ercentage2f+$percentage30+
            $percentage31+$percentage32+$percentage33+$percentage34+$p
+ercentage35+$percentage36+
            $percentage37+$percentage38+$percentage39+$percentage3a+$p
+ercentage3b+$percentage3c+
            $percentage3d+$percentage3e+$percentage3f+$percentage40+$p
+ercentage41+$percentage42+
            $percentage43+$percentage44+$percentage45+$percentage46+$p
+ercentage47+$percentage48+
            $percentage49+$percentage4a+$percentage4b+$percentage4c+$p
+ercentage4d+$percentage4e+
            $percentage4f+$percentage50+$percentage51+$percentage52+$p
+ercentage53+$percentage54+
            $percentage55+$percentage56+$percentage57+$percentage58+$p
+ercentage59+$percentage5a+
            $percentage5b+$percentage5c+$percentage5d+$percentage5e+$p
+ercentage5f+$percentage60+
            $percentage61+$percentage62+$percentage63+$percentage64+$p
+ercentage65+$percentage66+
            $percentage67+$percentage68+$percentage69+$percentage6a+$p
+ercentage6b+$percentage6c+
            $percentage6d+$percentage6e+$percentage6f+$percentage70+$p
+ercentage71+$percentage72+
            $percentage73+$percentage74+$percentage75+$percentage76+$p
+ercentage77+$percentage78+
            $percentage79+$percentage7a+$percentage7b+$percentage7c+$p
+ercentage7d+$percentage7e+
            $percentage7f+$percentage80+$percentage81+$percentage82+$p
+ercentage83+$percentage84+
            $percentage85+$percentage86+$percentage87+$percentage88+$p
+ercentage89+$percentage8a+
            $percentage8b+$percentage8c+$percentage8d+$percentage8e+$p
+ercentage8f+$percentage90+
            $percentage91+$percentage92+$percentage93+$percentage94+$p
+ercentage95+$percentage96+
            $percentage97+$percentage98+$percentage99+$percentage9a+$p
+ercentage9b+$percentage9c+
            $percentage9d+$percentage9e+$percentage9f+$percentagea0+$p
+ercentagea1+$percentagea2+
            $percentagea3+$percentagea4+$percentagea5+$percentagea6+$p
+ercentagea7+$percentagea8+
            $percentagea9+$percentageaa+$percentageab+$percentageac+$p
+ercentagead+$percentageae+
            $percentageaf+$percentageb0+$percentageb1+$percentageb2+$p
+ercentageb3+$percentageb4+
            $percentageb5+$percentageb6+$percentageb7+$percentageb8+$p
+ercentageb9+$percentageba+
            $percentagebb+$percentagebc+$percentagebd+$percentagebe+$p
+ercentagebf+$percentagec0+
            $percentagec1+$percentagec2+$percentagec3+$percentagec4+$p
+ercentagec5+$percentagec6+
            $percentagec7+$percentagec8+$percentagec9+$percentageca+$p
+ercentagecb+$percentagecc+
            $percentagecd+$percentagece+$percentagecf+$percentaged0+$p
+ercentaged1+$percentaged2+
            $percentaged3+$percentaged4+$percentaged5+$percentaged6+$p
+ercentaged7+$percentaged8+
            $percentaged9+$percentageda+$percentagedb+$percentagedc+$p
+ercentagedd+$percentagede+
            $percentagedf+$percentagee0+$percentagee1+$percentagee2+$p
+ercentagee3+$percentagee4+
            $percentagee5+$percentagee6+$percentagee7+$percentagee8+$p
+ercentagee9+$percentageea+
            $percentageeb+$percentageec+$percentageed+$percentageee+$p
+ercentageef+$percentagef0+
            $percentagef1+$percentagef2+$percentagef3+$percentagef4+$p
+ercentagef5+$percentagef6+
            $percentagef7+$percentagef8+$percentagef9+$percentagefa+$p
+ercentagefb+$percentagefc+
            $percentagefd+$percentagefe;
            

            
            print "0x00 percentage: $percentage00\n";
            print "0xFF percentage: $percentageff\n";
            my $average = $sum/254;                #divided by 254 bec
+ause i just wanted to average everything inbetween x00 and xff
            $average = sprintf("%.3f", $average);
            print "0x01 - 0xFE percentage: $average\n\n";
[download]

I tried to use eval with tr/// but it was returning unexpected results, and was probably something i was doing wrong. I did think about how i could do a loop or something, but the only way was with s///g and it took a while longer to accomplish what i was after. tho the script i made works great, yours is alot shorter and is faster.

Here's another approach that may be a little faster than my previous one (but it will be nowhere near 3 - 4 seconds for 256 MB!).

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'A man, a plan, a canal: Panama!';
 ;;
 my @char_counts;
 $#char_counts = 255;
 ++$char_counts[ ord(substr $s, $_, 1) ] for 0 .. length($s) - 1;
 die 'oops...' if $#char_counts != 255;
 ;;
 printf qq{'%s' (0x%02x) == $char_counts[$_] (%6.3f%%) \n},
   chr, $_, ($char_counts[$_] / length $s) * 100
   for grep defined($char_counts[$_]), 0 .. $#char_counts;
"
' ' (0x20) == 6 (19.355%)
'!' (0x21) == 1 ( 3.226%)
',' (0x2c) == 2 ( 6.452%)
':' (0x3a) == 1 ( 3.226%)
'A' (0x41) == 1 ( 3.226%)
'P' (0x50) == 1 ( 3.226%)
'a' (0x61) == 9 (29.032%)
'c' (0x63) == 1 ( 3.226%)
'l' (0x6c) == 2 ( 6.452%)
'm' (0x6d) == 2 ( 6.452%)
'n' (0x6e) == 4 (12.903%)
'p' (0x70) == 1 ( 3.226%)
[download]

... how HxD is able to do it ...

... is by writing the code in C or some such compiled language — at least, I'd be willing to bet doughnuts to dollars that's the case. You, too, can do this with Inline::C! (Update: See also Inline::C::Cookbook.) In fact, the array-based approach in the code example above should, I think, convert very neatly to C. The learning curve for Inline::C is not too bad (assuming you know C!) and well worth the effort if you have a need for speed! (I need to brush up on Inline::C myself, so if I have some time later, I may play around with this.)