"...but not 'CHECK_ABC123_A1.txt' "

Nope! Not true.

C:\>perl -E "my $s='CHECK_ABC123_A1.txt';say $s if ($s =~ m/^CHECK_(\w
++)\.txt/ );"
CHECK_ABC123_A1.txt
[download]

or, alternately,

#!/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
[download]

Pls test your advice... (unless you were trying to get a clueless OP to turn in homework with an error).