The co-ordinates start bottom left of page. Start a new page when your line position gets to the end of the page like this.

for my $item (@task) {
  my $msg = '[  ]  ' . $item;
  drawline( $page, $linepos );
  $txt->translate( 70, $linepos+10 );
  $txt->text($msg);
  $linepos -= $vspace;
  if ($linepos < 0){
    $page = $pdf->page;
    $txt  = $page->text;
    $txt->font( $font, 20 );
    $linepos = 650;
  }
}
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You might consider a newpage sub to repeat the heading and add a page number.

Sorry but I don't understand the question

poj