#!/usr/bin/env perl

use 5.010;

for (<>) {
    if (/^>/) {
        # Header
    } elsif (/^[A-Z]+$/) {
        # Protein
        my $a = tr/A/A/;
        say "A: $a, length: " . length;
    }
}
~                                    

##</code><code>##

>sp|P76347|YEEJ_ECOLI Uncharacterized protein YeeJ OS=Escherichia coli (strain K12) OX=83333 GN=yeeJ PE=3 SV=3
MATKKRSGEEINDRQILCGMGIKLRRLTAGICLITQLAFPMAAAAQGVVNAATQQPVPAQ
IAIANANTVPYTLGALESAQSVAERFGISVAELRKLNQFRTFARGFDNVRQGDELDVPAQ
VSEKKLTPPPGNSSDNLEQQIASTSQQIGSLLAEDMNSEQAANMARGWASSQASGAMTDW
LSRFGTARITLGVDEDFSLKNSQFDFLHPWYETPDNLFFSQHTLHRTDERTQINNGLGWR
HFTPTWMSGINFFFDHDLSRYHSRAGIGAEYWRDYLKLSSNGYLRLTNWRSAPELDNDYE
ARPANGWDVRAESWLPAWPHLGGKLVYEQYYGDEVA