use String::CRC32;
my $initial_str = "foo bar";
my $desired_crc = 0x12345678;
my $magic = forge_crc(value => $desired_crc ^ 0xffffffff)
          ^ crc32($initial_str) ^ 0xffffffff;
my $forged_str = $initial_str . pack("V", $magic);
printf "%x\n", crc32($forged_str);

sub forge_crc {
  my (%opts) = @_;
  # default to the usual crc32 polynomial
  my $poly    = $opts{poly} || 0xedb88320;
  my $size    = $opts{size} || 32;
  my $highbit = 1 << ($size-1);
  my $reg     = $opts{value};
 
  $poly = ($poly << 1) ^ 1;
  for (1..$size) {
    $reg = ($reg & $highbit) ?
        ($reg << 1) ^ $poly : $reg << 1;
  }
  return $reg;
} # forge_crc