It looks like the command is returning -1:

    perl -le 'print -1 >> 8'
    16777215
[download]

Which may indicate that the command isn't valid:

    perl -le '`foobar`'
    -1
[download]

Or if the command is valid, then perhaps the return value of -1 indicates that your program doesn't have permission or resources to run it.

--
John.