#! perl
use strict;
use warnings;
use Tie::File;

my $filename = 'RawFile.txt';

tie my @lines, 'Tie::File', $filename or die "Cannot tie file '$filename': $!";

for my $i (0 .. $#lines)
{
    my @fields = split /\t/, $lines[$i], -1;
    @fields = map { $_ eq '' ? 'Missing' : $_ } @fields;
    $lines[$i] = join("\t", @fields);
}

untie @lines;

##</code><code>##

abcd    123     456     defg
cdefg   23      Missing as
Missing 345     235     Missing
xsd     Missing swe     Missing