Whatever is building the command line is doing it wrong:

-passin pass:'test me'
[download]

... should likely be

-passin 'pass:test me'
[download]

For example, the following line building some command line will also not deal well with spaces in your password:

$cmd = "-des3 -passout pass:$args->{keypass} ".$cmd if defined($args->
+{keypass});
[download]

Also, this line:

my $common_args = "-$args->{digest} -days $args->{days} ".
" -key $cakey -passin pass:$args->{keypass}";
[download]

As a general rule, if it doesn't work on the command line/shell prompt, there is very little that IPC::Run can do to make it work either.