Any clues why and how can I do that?

Yes. You are giving perl a string and nothing more. Perl looks at the string and happily throws it away.

# string
"system(/usr/bin/perl /root/test.pl)"

# program
system "/usr/bin/perl /root/test.pl";
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So, omit the double quotes; try instead

perl -e 'system "/usr/bin/perl /root/test.pl"'
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or this

perl -e 'system "/usr/bin/perl","/root/test.pl"'
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See system for the difference between the two expressions.