$check ="ABCD" # this value will come from an array 
        # ...
                $var = substr($file_name, 0, 3);
                # ...
                if($var eq $check){

##</code><code>##

sub scandir {
    my $dir=shift;
    my $test=join '|', map quotemeta, @_;
    chdir $dir;
    for (glob '*') {
        next unless /^($test)(.*)/;
        my $new="$1D$2";
        print "`$_' => `$new'\n";
        if (-e $new) {
            warn "Warning: `$new' exists\n";
            # or do something more complex...
            next;
        } else {
            rename $_, $new or die "Can't rename to `$new': $!\n";
        }
    }
}