Take a look at Perl system function.
I don't understand capture_merged{} in the context of your code.
What is "capture_merged"?.
I don't see an open to the file handle $log.

When debugging an error, simplify, simplify. See how you get on with:

my @args = ("command", "arg1", "arg2");
system(@args) == 0 
     or die "system @args failed: $?";
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Single quotes, ' vs double quotes, " matter.