So why can't they go just about anywhere before another delimiter, since everyting is a (sub) expression?

Semi-colons can be put after ever statement, but not every expression. If you were to put semi-colons after every expression the you couldn't build up larger expressions e.g

#    exp
#    exp             and   exp
# ( (exp || exp)     and  (exp || exp))
   foo() || $bar->() and  $baz || @quux;
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print *{   # beginning of glob dereference
  ${       # beginning of scalar derference
     $x    # scalar statement to be dereferenced
   }       # end of scalar dereference
   {$y}    # glob statement to be dereferenced (+ ${$x} )
}          # end of glob statement to be dereferenced
{SCALAR}   # glob slot and end of print statement
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_________
broquaint

¹ I Am Not A Language Guru