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


##</code><code>##

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