Well, shouldn't I feel stupid now? Of course, I had to post my stupidity for all to see before I could find what was plainly in sight. How typical!

Just for the sake of anybody who might be misled by my question above, the style that introduces a new page layout should only be applied to a single paragraph and it seems to remain effective for the rest of the document (unless, of course, you need to apply a new page layout). All following paragraphs should have a style that is not linked with any page layout.

So, here's the corrected code that does the job properly:

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

use utf8;
use Win32;
use Encode;
use OpenOffice::OODoc;
use Data::Dumper::AutoEncode;

ooLocalEncoding('utf-8');

my $curfolder = Win32::GetCwd();
my $outfolder = $curfolder . '\\out';
my $outfile   = $outfolder . '\\test.odt';

my $doc = odfDocument(
    file         => $outfile,
    create       => 'text',
    opendocument => 0,
);
my $styles = odfDocument(
    container => $doc,
    part      => "styles",
);
my $centerfooter = $styles->createStyle(
    'centerfooter',
    family     => "paragraph",
    properties => {
        "fo:margin-top" => "0.5cm",
        "fo:text-align" => 'center',
    },
    replace => 1,
);
my $headerstyle = $styles->createStyle(
    "header",
    family     => "paragraph",
    parent     => "Standard",
    properties => {
        "fo:margin-top"  => "8cm",
        "fo:text-align"  => "center",
        "fo:break-after" => "page",
    },
    replace => 1,
);
$styles->styleProperties(
    $headerstyle,
    -area            => "text",
    "fo:font-size"   => "200%",
    "fo:font-weight" => "bold",
);
$styles->setAttributes(
    $headerstyle,
    "master-page-name" => 'header',
);
my $regularnextstyle = $styles->createStyle(
    "regularnext",
    family  => "paragraph",
    parent  => "Standard",
    replace => 1,
);
my $regularstyle = $styles->createStyle(
    "regular",
    family  => "paragraph",
    parent  => "regularnext",
    replace => 1,
);
$styles->setAttributes(
    $regularstyle,
    "master-page-name" => 'pagenumbers',
);

my $pagelayout = $styles->pageLayout("Standard");
my $titlepage = $styles->createMasterPage(
    'titlepage',
    layout => $pagelayout,
);

my $pnpage = $styles->createMasterPage(
    'pagenumbers',
    layout => $pagelayout,
);
my $pn = $styles->createParagraph( '', 'centerfooter' );
my $pg = $styles->textField( 'page-number', style => 'centerfooter' );
$styles->appendElement( $pn, $pg );
$styles->masterPageFooter( 'pagenumbers', $pn );

my $wordlist = $doc->appendParagraph(
    text  => '',
    style => 'header',
);
$doc->extendText( $wordlist, uc 'Main Title', 'header' );
my $regular = $doc->appendParagraph(
    text  => 'Some text.',
    style => 'regular',
);
$doc->appendParagraph(
    text  => 'More text.',
    style => 'regularnext',
);
for (my $i = 0; $i < 100; $i++) {
    $doc->appendParagraph(
        text  => 'And some more.',
        style => 'regularnext',
    );
}
$doc->save;
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