Try the following, which uses a zero-width negative lookahead for the "Z":

use warnings;
use strict;

while (<DATA>){
    chomp;
    print "$_ matches\n" if /ABC(?!Z)/; 
}

__DATA__
ABC
ABCD
ABCZ
ABCZADF
ABCDZADF
AB
XFC
123
AB1
ABC1
XXXABCZ2
ZZZABCDZ2

__END__
ABC matches
ABCD matches
ABCDZADF matches
ABC1 matches
ZZZABCDZ2 matches
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You didn't state where in a string this can match, so this'll match anywhere.