$tcount;
$acount;
$gcount;
$ccount;
foreach $letter (@DNA)
{
   if (($letter eq "t" ) || ($letter eq "T"))
   {
         $tcount++:
}
and so on

#!/usr/bin/perl -w
#CountLetters.pl
use strict;
print "Enter string of letters: ";
chomp (my $input = <STDIN>);
my ($char, $count, $verb, $s);
print "\n";
while ($input =~ m/(.)/g) {
    $char = $1;
    #Count and remove all instances of this character -- lower and uppercase
    $count = $input =~ s/$char//gi;
    #Did we find more than one? Singular or plural?
    $verb = $count == 1 ? "is" : "are";
    $s = $count > 1 ? "s" : "";
    print "There $verb $count '$char'$s.\n";
}

#!/usr/bin/perl -w
#CountLetters.pl
use strict;
print "Enter string of letters: ";
chomp (my $input = <STDIN>);
my ($char, $count, $verb, $s, %hash);
print "\n";
 
while ($input =~ m/(.)/) {
    #When all characters have been counted and replaced with Null,
    # $input will not match /(.)/ and the loop will end
    $char = $1;
    #Count and remove all instances of this character -- lower and uppercase
    #Replace $char with Null so we won't count same character again
    $count = $input =~ s/$char//gi; 
    #Did we find more than one? Singular or plural?
    $verb = $count == 1 ? "is" : "are";
    $s = $count > 1 ? "s" : "";
    print "There $verb $count '$char'$s.\n";
    $hash{$char} = $count; #Store $char as hash key and $count as hash value
}
print "\nThe letter counts have been stored in a hash.\n";
while (($char, $count) = each %hash ){
	print "Hash key '$char' has hash value $hash{$char}\n";
}

#!/usr/bin/perl
use Data::Dumper;
 
print "Enter String:\n";
my $string=<STDIN>;
chomp($string);
%hash;
my @a=split(//,$string);
foreach(@a)
{
        if(defined $hash{$_})
        {
                $i=$hash{$_};
                $i++;
                $hash{$_} = $i;
        }
        else{
                $i=1;
                $hash{$_}=$i;
        }
 
}
print Dumper \%hash;exit;