You could use negative lookahead. An example:

use strict;
use warnings;

my @lines = <DATA>;
my $data;
foreach $data (@lines) {
    if($data =~ /sometext (?!not)\w* endtext/) {
        print "$data passed \n";
    }
}

__DATA__
sometext not endtext
sometext positive endtext
[download]

outputs

sometext positive endtext
 passed
[download]