use Modern::Perl;
while (<DATA>)
{
    chomp;       # remove trailing line break
    say "original string is:  '$_'";
    
    s/ATGC.*$//; # strip 'ATGC' and anything following it
    say "processed string is: '$_'";

    say "----";
}
__DATA__
123ATGC
123ATGC456
ATGC123
123ATGC456ATGC789
ATGC
12345