use strict;
use warnings;

use 5.014;

open my $inFH, q{<}, \ <<EOD or die $!;
atggctgctaggctgtgctgccaactggatcctgcgcgggacgtcctttgtctacgtcccgtcggcgctgaatcctgcggacgacccctctcgtggtcgcttggggctctgccgccctcttctccgcctgccgttccggc
atagctgctaggctgtgctgccaactggatcctgcgcgggacgtcctttgtctacgtcccgtcggcgctgaatcctgcggacgacccctctcgtggtcgtttggggctctgccgccctcttctccgcctgccgttcaggc
atggctgctaggctgtgctgccaactggatcctgcgcgggacgtcctttgtctacgtcccgtcggcgctgaatcctgcggacgacccctctcgtggtcgcttggggctctgccgccctcttctccgcctgccgttccggc
atggctgctaggctgtgctgccaactggatcctgcgcgggacgtcctttgtctacgtcccgtcggcgctgaatcctgcggacgacccctctcgtggtcgcttggggctctaccgccctcttctccgcctgccgttccggc
EOD

chomp( my @sequences = <$inFH> );
close $inFH or die $!;

my @letters   = map { [ split m{} ] } @sequences;
my $consensus = q{};

while ( @{ $letters[ 0 ] } )
{
    my $seq0 = shift @{ $letters[ 0 ] };
    my $seq1 = shift @{ $letters[ 1 ] };
    my $seq2 = shift @{ $letters[ 2 ] };
    my $seq3 = shift @{ $letters[ 3 ] };
    if (
       $seq0 eq $seq1
       and
       $seq0 eq $seq2
       and
       $seq0 eq $seq3
       )
    {
        $consensus .= $seq0;
    }
    else
    {
        emitConsensus();
    }
}
emitConsensus();

sub emitConsensus
{
    say $consensus if length $consensus;
    $consensus = q{};
}