use FindBin;
use File::Spec;
use lib File::Spec->catdir($FindBin::Bin, 'lib'); # get modules from the lib subdirectory under this perl script

#...

my $exe = File::Spec->catfile($FindBin::Bin,
   File::Spec->updir(),
   qw(PDFtoHTML pdftohtml));
system($exe, $pdffile) == 0 or do {
  # handle system error.
}