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my @fields = ... ;

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

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

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