my @fields = ... ;

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

##</code><code>##

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

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

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