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...

use strict;
use warnings;

my $header_line = <DATA>;
my %count;
my @chars = ( qw/F G S/ );

while ( my $line = <DATA> ) {
   foreach ( @chars ) {
      my $pos = 0;
      while ( ($pos = index( $line, $_, $pos ) ) >= 0) {
         $count{$_}++;
         $pos++;
      }
   }
}


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

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

Dave

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