One's an assignment of the result of a substitution on the default argument $_ (that result being the number of substitutions made), the other is specifying the target of a substitution.

Update: expanded on what the result would be in the first case.

The cake is a lie.
The cake is a lie.
The cake is a lie.

In reply to Re: question on regexp: by Fletch
in thread question on regexp: by roc

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