join takes a list as its second argument, but you are passing a string.

What it appears you want to do is:

print scalar reverse,' ' for qw[teY rehtona lreP .rekcah];
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The qw// creates the list, and then for each element, prints the reverse of the string (thus the 'scalar'). Or, you could try:

print scalar reverse,' ' for split ' ', q[teY rehtona lreP .rekcah];
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If you really wanted it to be a string.