A little experimenting revealed that the text is searchable if it is in the non-TTF fonts (i.e. selected with prFont) and not searchable if it is in an external TTF font (i.e. selected with prTTFont).

The following code prints each line twice. The first one can be searched for, but greek text is uninteligible, while the second line, displays greek text fine, but none of it can be searched for

#!/usr/bin/perl
use warnings;
use strict;
use PDF::Reuse;
use Encode;

prFile('test.pdf');

#reading UTF-8 data from file
open FILE, "<testFile.txt" or die "$!\n";;

my $content = "";
my $y = 760;
while (<FILE>)
{
   chomp;
   $content = decode('CP1253', $_); #My test file is in Windows Greek

   prFont('H');
   prText(100, $y, $content );
   $y-=12;
   prTTFont('arialuni.ttf');
   prText(100, $y, $content );
   $y-=40;
}
prEnd();

close FILE;
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I hope someone can understand why this is so, because the text I want to write to a pdf is in Greek and there does not seem to be any solution for me otherwise.