What triggers the Ruby warning about ambiguous first argument?

In Ruby 1.9.1, if you do

$VERBOSE = true
puts /m/ , 42.to_s

or if I do

$VERBOSE = true
puts /m/ , "42"

You get the warning

warning: ambiguous first argument; put parentheses or even spaces

But I don't get it if I do

$VERBOSE = true
puts "m" , 42.to_s

$VERBOSE = true
puts(/m/, 42.to_s)

So what specifically triggers this warning? And what more spaces could I have added to the original expression?

ruby warnings

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