This is slightly easier on (my) eyes...

my $text = <<END;
create table foo (
   somevar1 sometype,
   somevar2 sometype null,
   somevar3 sometype not null,
   somevar4 sometype(20),
   somevar5 sometype(20) null,
   somevar6 sometype(10))
END

my $header  = qr/create\s+table\s+\w+\s*\(\s*/;
my $name    = qr/\w+\s*/;
my $type    = qr/\w+\s*/;
my $op_char = qr/(?:\(\d+\)\s*)?/;
my $words   = qr/(?:\w+\s*)*/;
my $decl    = qr/$name $type $op_char $words/x;
my $comma   = qr/,\s*/;
my $rest    = qr/\)\s*/;

$text =~ s{($header)
           ((?: $decl $comma)*
            $decl)
           ($rest)}
          {
              $1 . (join ",", 
                      map {$_ . (/null\s*$/ ? "" : " NOT NULL")}
                          split(/,/, $2)) . $3
          }gex;

print $text;
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