my $header_line = <DATA>;

my %count;

my @chars = ( qw/F G S/ );

while (my $line = <DATA> ) {
    eval "\$count{$_} += \$line =~ tr/$_/$_/,1" 
    or die $@ foreach @chars;
}

print "There are $count{$_} occurrences of $_\n"
foreach sort keys %count;

__DATA__
Sample header hine
FDIELSIGCOXLSAGICK\n
FDIELSIGCOXLSAGICK\n
[download]

The reason that the tr/// must appear inside of an eval block is that variables are not interpolated in tr/// (the transliteration table is built at compiletime, not runtime). Eval forces a fresh compilation of tr/// each time through the loop.

The reason that I pass references is because I want the variables to exist as variables inside the eval, not as values (except in the case of what's inside the tr/// itself).

And the '1' appears at the end of the eval expression so that eval returns safely (without croaking) even if no matches are found.

I think this is an elegant solution, and saves a lot of intricate fiddling.

If you want to see a solution that uses index instead of tr///, you may...

Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein

    my  $temp = <DATA>;
    print "Discarding the file header line.\n";
    print "    ", $temp;
    
    my  @chars = qw( A C G T );
    my  %cnts;
    while( <DATA> ) {
        foreach my $char (@chars) {
#           my  $cnt = () = m/$char/g;
#           $cnts{$char} += $cnt;
#           printf "  For char '%s' I found %d\n", $char, $cnt;
            $cnts{$char} += () = m/$char/g;
        }
    }
    foreach my $char (@chars) {
        printf "  Char '%s':  %6d\n", $char, $cnts{$char};
    }

__DATA__
Generated by a completely confused program yesterday
ACGTGACTAGAGGCCCGGGGAAAAAAAAAACCCCCCC
ACCTGACTAGAGGCCCGGGGAAAAAAAAAACCCCCCC
ACGTGACTAGAGGCCCGGGGAAAAAAAAAACCCCCCC
AGGTGAGTAGAGGGGGGGGGAAAAAAAAAAGGGGGGG
ACGTGACTAGAGGCCCGGGGAAAAAAAAAACCCCCCC
[download]

Discarding the file header line.
    Generated by a completely confused program yesterday
  Char 'A':      70
  Char 'C':      49
  Char 'G':      56
  Char 'T':      10

Since we're spewing bilge today, thought I'd post what I'd been thinking to do to avoid the RE recompiles.   I know others have done it (many times) before, but ...

    my  $magic = 'sub { ';
    foreach my $char (@chars) {
        $magic .= "\$cnts{$char} += ()= \$_[0] =~ m/$char/g;";
    }
    $magic .= '}';

    my  $magicsub = eval "$magic";
    die "eval to create anon sub failed '$@'"
        unless defined $magicsub;

    while( <DATA> ) {
        $magicsub->($_);
    }
[download]


And in honor of the day:
@pieces=split //, 'eight'; print "@pieces squawk!\n" for(1..2);

<FILE>; # ignore first line
my @words = (A, C, G, T); #letters to search
my @file = do {local $/; split (//, <FILE>); }; #split each letter int
+o an array

foreach $file (@file){
    foreach $words (@words){
        if ($file eq $words){
             $word_list{$file}++;
        }
    }
}
@pairs = sort {$a->[1] <=> $b->[1]}
         map {[ $_ => $word_list{$_}]}
         keys %word_list;

print "word $_->[0] = $_->[1]\n" for @pairs;
[download]