The reason is that the look-ahead is not limited to what the [\w\s]{49,}? matches. A small demonstration:

#!/usr/bin/perl
use strict;
use warnings;

my $re = qr{^\s*(?=\w+\s+\w+)[\w\s]{49,}?\w};

my $str =  ('x' x 65) . ' x';

if ($str =~ m/$re/) {
    print $&, $/;
}
__END__
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You see that match only includes a long word, not a sentence. There is a sentence present, but it's not matched.

Update: In Perl 6 you could use & like this:

regex short_word {
   \s* 
   [ 
   & \w+ \s+ \w+ 
   & .**?{1..50}
   ] 
}
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