it shows that the perl is using only one core

This really is most unlikely. It seems far more likely that the tools you are using to monitor the usage, or your interpretation of that information is at fault.

Though it also seems unlikely that the implementation of a system tool like top on a modern OS could be quite so broken as suggested elsewhere.

Something as simple as:

perl -Mthreads -E"async{ my $x; ++$x for 1 .. 100e6 }->detach for 1 ..
+8; sleep 10"
[download]

Should red-line all your cpus for a few seconds. If it doesn't, you're either misinterpreting the tools or your perl installation is very broken, but I'd expect some errors if it was the latter.