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
[download]

produces output...

original string is:  '123ATGC'
processed string is: '123'
----
original string is:  '123ATGC456'
processed string is: '123'
----
original string is:  'ATGC123'
processed string is: ''
----
original string is:  '123ATGC456ATGC789'
processed string is: '123'
----
original string is:  'ATGC'
processed string is: ''
----
original string is:  '12345'
processed string is: '12345'
----