my $seq=(keys %SEQ)[$j]   <--- missing semicolon

##</code><code>##

    for(my $i=1; $i<@aas; $i++){
        my $aa = $aas[$i];
        @codons = @{$Deg_codons{$aa}};
        my @SEQ = keys %SEQ;
        my $s = scalar @SEQ;
        for(my $j=0;$j<$s;$j++){
            my $seq=@SEQ[$j];
            foreach my $codon(@codons){
                $SEQ{$seq.$codon}=1;
                $j++;
                $s++;
            }
            delete $SEQ{$seq};
            $j--;
            $s--;
        }
    }