You're re-opening $out_pdf for every new input file, that may be one of the reasons. Try to open output pdf file just once before the while loop.

That certainly is a problem. However, according to the PDF::API2 docs for the update method, $out_pdf is removed from memory after the first iteration of the loop after writing out to the merged pdf.

$pdf->update()
    Saves a previously opened document.

<...>

$pdf->end()

    Remove the object structure from memory. PDF::API2 contains circul
+ar references, so this call is necessary in long-running processes to
+ keep from running out of memory.

    This will be called automatically when you save or stringify a PDF
+.
[download]

I get the following error when I modify the script to open $out_pdf before the loop:

The new, updated script follows:

#!/usr/bin/perl
use 5.018;
use PDF::API2;
use strict;
use warnings;

my $path = "./pdfs/";
my $out_pdf_file = 'merged.pdf';
my $out_pdf = PDF::API2->new(-file => $out_pdf_file);

opendir (my $DIR, $path) or die "Could not open $path:\n$!\n$^E";
chdir $path;

while ( my $in_pdf_file = readdir $DIR ) {
  next if $in_pdf_file =~ /^\./;
  
  my $in_pdf = PDF::API2->open($in_pdf_file) or die "Error opening PDF
+ file [$in_pdf_file]:\n$!\n$^E";
  
  foreach my $page ( 1 .. $in_pdf->pages() ) {
    $out_pdf->import_page($in_pdf, $page, 0);
  }
  $out_pdf->update();
}

closedir $DIR;
[download]

Moving $out_pdf->update(); out of the loop fixes the undefined value error, but the script quickly exhausts all the memory on the computer.

If you don't need any PDF::API2 specific features, then perhaps you can try something else? For example CAM::PDF. It comes with appendpdf.pl script which does something similar to what you need.