#!/usr/bin/perl
use strict;
use warnings;

print "Enter the file containing the sequence: ";
my $filename = <STDIN>;
chomp $filename;

open my $FH, '<', $filename or die "Cannot open $filename: $!";

my @chars = qw(A C D E F G H I K L M N P Q R S T V W Y);
my $length;
my $char_regex = join q(), @chars;
$char_regex = qr/[$char_regex]/;

my %occ;

while (my $line = <$FH>)
{
    for my $char (split //, $line)
    {
        next unless $char =~ $char_regex;
        $length++;
        $occ{$char}++;
    }
}

print "AMINO ACID \t OCCURRENCE \t FREQUENCY\n";

for my $char (@chars)
{
    $occ{$char} //= 0;
    print  "$char \t\t $occ{$char} \t\t ", $occ{$char} / $length, "\n";
}