Another way. For input entered by a user (even you!), it is usually very important to verify the input. Note that in a statement like
    Readonly my $RX_DNA => qr{ [ATCGatcg]+ }xms;
a => (fat comma) is used instead of an = (assignment) operator. If you don't want to use Readonly, make the statement
    my $RX_DNA = qr{ [ATCGatcg]+ }xms;
instead.

>perl -wMstrict -e
"use Readonly;
 ;;
 Readonly my $RX_ID  => qr{ \d+         }xms;
 Readonly my $RX_DNA => qr{ [ATCGatcg]+ }xms;
 ;;
 my %pairs;
 print qq{please enter a comma-separated id/dna pair: };
 PAIR:
 while (chomp(my $pair = <STDIN>)) {
   last PAIR unless $pair;
   my ($id, $dna) =
     $pair =~ m{ \A ($RX_ID) \s* , \s* ($RX_DNA) \z }xms;
   if (defined $id) {
     $pairs{$id} = $dna;
     }
   else {
     print qq{'$pair' unrecognizable: rejected. \n};
     }
   print qq{please enter another id/dna pair: };
   }
 ;;
 use Data::Dumper;
 print Dumper \%pairs;
"
please enter a comma-separated id/dna pair: 123, ata
please enter another id/dna pair: 222 cgcg
'222 cgcg' unrecognizable: rejected.
please enter another id/dna pair: 222, cgcg
please enter another id/dna pair: 33, abcd
'33, abcd' unrecognizable: rejected.
please enter another id/dna pair: 33 ,acGT
please enter another id/dna pair:
$VAR1 = {
          '123' => 'ata',
          '33' => 'acGT',
          '222' => 'cgcg'
        };
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