I have some 6 field say field 1,field 2,field 3,field 4,field 5,field 6.

You'll have to clarify what you mean by "field"?

Are these substrings within a single string? Or entryfields within an HTML form? Columns in a CSV file?

And also what you mean by "blank"?

Let's say that you have captured the fields into an array, then you might build you binary value this way:

my @fields = ... ;

my $binary = chr(0);
defined( $fields[ $_ ] ) && length( $fields[ $_ ] ) and vec( $binary, 
+$_, 1 ) = 1 for 0 .. $#fields;
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For example:

@fields = ( 'fred', undef, '', 0, 123, '  ' );;
$binary = '';;

vec( $binary, $_, 1 ) = ($fields[ $_ ] // '' ) =~ /\S/ ? 1 : 0 for 0 .
+. $#fields;;

print unpack 'b*', $binary;;
10011000
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