use strict; 
use warnings; 
my @files = ( 
    'ABC1.txt', 
    'ABC1a_1.txt', 
    'ABC1a_1_1.txt', 
    'ABC1b.txt', 
    'ABC1b_1.txt', 
    'ABC11.txt', 
    'ABC1.png', 
); 
 
my @special_files = grep {/ABC1(:?a_1|a_1_1|b|b_1)?\.txt/} @files; 
print "@special_files\n";