From 'perldoc perldiag':

Substitution loop
 (P) The substitution was looping infinitely.
 (Obviously, a substitution shouldn't iterate more
 times than there are characters of input, which is
 what happened.)  See the discussion of substitution in
 the section on Quote and Quote-like Operators in the
 perlop manpage.
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What is the value of $sep when you get this warning?

Cheers,

- danboo