This may work: (UNTESTED)
open my $out_file, '>', 'aa_report.txt' or die "Cannot open 'aa_report
+.txt' because: $!";
print 'PLEASE ENTER THE FILENAME OF THE PROTEIN SEQUENCE: ';
chomp( my $prot_filename = <STDIN> );
open my $PROTFILE, '<', $prot_filename or die "Cannot open '$prot_file
+name' because: $!";
$/ = ''; # Set paragraph mode
while ( my $para = <$PROTFILE> ) {
# Remove fasta header line
$para =~ s/^>.*//m;
# Remove comment line(s)
$para =~ s/^\s*#.*//mg;
my %prot;
$para =~ s/([A-Z])/ ++$prot{ $1 } /eg;
print $out_file join( ' ', map "$_=$prot{$_}", sort keys %prot ),
+"\n";
}