#!/usr/bin/perl
use 5.016;
use strict;
use warnings;

# 1042260

my $s='CHECK_ABC123_A1.txt';
if ( $s =~ m/^CHECK_(\w+)\.txt/ ) {
    say "\t \$1 is: $1\n\t and the regex matched the source string.";
} else {
    say "\t Nope. Regex didn't match \$s.";
}

=head OUTPUT:


         $1 is: ABC123_A1
         and the regex matched the source string.

=cut