/^CHECK.*[.]txt$/

##</code><code>##

/^CHECK.*(?<!A1)[.]txt$/

##</code><code>##

$ perl -Mstrict -Mwarnings -E '
    my @test_filenames = qw{
        CHECK_CH.ABC12_A1.txt
        CHECK_CH.ABC12.txt
        HECK_CH.ABC12_A1.txt
        HECK_CH.ABC12.txt
        CHECK_CH.A1.txt
        CHECK_CH..txt
        CHECK.txt
        CHECKA1.txt
        CHECK.ABC12.txt
    };

    say "*** May be nothing between CHECK & .txt ***";
    for (@test_filenames) {
        next unless /^CHECK.*(?<!A1)[.]txt$/;
        say;
    }

    say "*** Must be something between CHECK & .txt ***";
    for (@test_filenames) {
        next unless /^CHECK.+(?<!A1)[.]txt$/;
        say;
    }
'
*** May be nothing between CHECK & .txt ***
CHECK_CH.ABC12.txt
CHECK_CH..txt
CHECK.txt
CHECK.ABC12.txt
*** Must be something between CHECK & .txt ***
CHECK_CH.ABC12.txt
CHECK_CH..txt
CHECK.ABC12.txt