If "highlighted text" is done, as it should, with "colored overlay", then you extract highlights' positions like this:

use strict;
use warnings;
use feature 'say';
use CAM::PDF;

my $pdf  = CAM::PDF-> new( $ARGV[ 0 ]) or die;
my $page = $pdf-> getPage( 1 );
my $anns = $pdf-> getValue( $page-> { Annots } or die );
for ( @$anns ) {
    my $ann = $pdf-> getValue( $_ );
    next unless $pdf-> getValue( $ann-> { Subtype }) eq 'Highlight';
    say $ann;
    say "\t$_" for map $pdf-> getValue( $_ ), 
        @{ $pdf-> getValue( $ann-> { QuadPoints })}
}

__END__

HASH(0xd79f0c)
        237.641
        651.308
        271.059
        651.308
        237.641
        641.602
        271.059
        641.602
        61.4118
        637.963
        92.1406
        637.963
        61.4118
        628.257
        92.1406
        628.257
HASH(0xe8f43c)
        288.529
        611.271
        320.753
        611.271
        288.529
        601.566
        320.753
        601.566
[download]

Large pdf. They still didn't fix wrong order of points in that picture, take care. Also extract xml with each character bounding box coordinates. Doing it with pure Perl is possible, but involves too much low level work. I prefer mutool (mudraw, if older versions are packaged for your OS) and its "stext" output, GS might also do, adjust for (0,0) being upper left page corner. Walk over character nodes, start extracting text when BB is inside any quad, until you leave that quad. Continue till page end. It's really easy.