use strict;
use warnings;

use Data::Dumper;  # for debug

use constant {  # regexes to check and control input
    BLANK_LINE => qr{ \A \s* \Z }xms,
    NON_SEQ    => qr{ [^[:alpha:]] }xms,  # could be ATCG or IUPAC
    };

use constant USER_PROMPTS_IN_ORDER => qw(first secundus third 4th);

my %prompted_seqs;  # collected info on prompted sequences entered by user

print "please enter DNA sequences: \n";
PROMPT:
for my $prompt (USER_PROMPTS_IN_ORDER) {
    print "  $prompt sequence: ";
    my $input = <STDIN>;
    chomp $input;
    print("  stop getting sequences: user request \n"), last PROMPT
        if $input =~ BLANK_LINE;
    print("  stop getting sequences: bad input: '$input' \n"), last PROMPT
        if $input =~ NON_SEQ;
    $prompted_seqs{$prompt}{seq} = $input;
    $prompted_seqs{$prompt}{len} = length $input;
    }

print Dumper \%prompted_seqs;  # for debug

print "valid entered sequences in order of prompt: \n";
print "  $_: $prompted_seqs{$_}{seq} ($prompted_seqs{$_}{len}) \n"
    for grep { exists $prompted_seqs{$_} } USER_PROMPTS_IN_ORDER;

print "valid entered sequences in order of increasing length: \n";
print "  $_: $prompted_seqs{$_}{seq} ($prompted_seqs{$_}{len}) \n"
    for sort { $prompted_seqs{$a}{len} <=> $prompted_seqs{$b}{len} }
        keys %prompted_seqs
        ;