A /g modifier on a replace (s///) operator replaces all matches (and thus removes all [more] substrings, so there's nothing to match for the following iterations). Leave the /g modifier out, and it will replace the first match (leaving the rest to be replaced by the next match(es)).

See the section on s/PATTERN/REPLACEMENT/egimosx in perlop:

 Options are:

                   e   Evaluate the right side as an expression.
                   g   Replace globally, i.e., all occurrences.
                   i   Do case-insensitive pattern matching.
                   m   Treat string as multiple lines.
                   o   Compile pattern only once.
                   s   Treat string as single line.
                   x   Use extended regular expressions.
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