Perhaps it would help to think of it like this -- print is print (an expression). Like "print (2+2)" will print 4. So your first example isn't a value or a string, it's an expression that gets evaluated.

print ($string =~ s/_/ /g);
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.”
M-J D