#!/usr/bin/perl
use 5.014;
# 991184

my @positions;
my @data = ('1 ACAC',
            '2 AGAC',
            '3 AGTC',
            '4 ACCA',
            '5 DUMMY',
            '6 ACAATG',
            '7 CAAC',
            '8 acaacc',
            '9 aca ',
            );

for my $data(@data) {
    if ( $data =~ /[GT]/ || $data =~ /[^[ACTG]{4}$/) {
        say "Data ERROR or contains G or T ( $data )";
        next;
    } elsif ( $data =~  /^\d+ [AC]{4}$/i ) { # too many; too few? covered here
            $data =~ s/ [AC]{4}//i;
            chomp $data;
            push @positions, $data;
    } else {
            say "Problem with data? $data";
    }
}

print "\n Good data at positions: ";
for my $position (@positions) {
    print "$position ";          # depending on size of valid positions, you may 
}                                # want to stack them vertically --
                                 # simply replace the 'print' with 'say'
say "\n Done";