my $FH = "DATA"; open $FH, "<", $filename or croak "Cannot open file $filename $!\n";

##</code><code>##

open my $FILE, '<', $filename
    or croak "Can't open '$filename': $OS_ERROR";

##</code><code>##

    my $FH;
    open $FH, "<", $filename     
        or croak "Cannot open file $filename $!\n";