I am a beginner too, but here is my attempt. It is very quick and very dirty (since it takes header and footer from each slide as text as well as the text) but it does extract the text from the pptx (at least from my pptx :-) ) and perhaps it could be useful.

#!/perl
use strict;
use warnings;

use Archive::Zip qw( :ERROR_CODES );
use XML::Twig;
my @text;

my $file = "The full path to your pptx";
my $zip = Archive::Zip->new();
$zip->read( $file ) == AZ_OK or die "Unable to open Office file\n";
my @slides = $zip->membersMatching( "ppt/slides/slide.+\.xml" );

for my $i ( 1 .. scalar @slides ) # to sort them.
{
    my $content =  $zip->contents( "ppt/slides/slide${i}.xml");
    my $twig= XML::Twig->new( #keep_encoding=>1,
        twig_handlers => { 'a:t' => \&topicref_processing,
            },
        );
    $twig->parse( $content );
}

sub topicref_processing 
{
    my($twig, $ppttext) = @_;
    push @text, $ppttext->text();
}

use FindBin qw($Bin);
open my $out, ">:encoding(UTF-8)", "$Bin/test_ppt_PM.txt" or die "$!";
print {$out} "$_\n" for @text;
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