From the Perl Cookbook, recipe 6.1.Copying and Substituting Simultaneously:

You can even use this technique on an entire array:

@bindirs = qw( /usr/bin /bin /usr/local/bin );
for (@libdirs = @bindirs) { s/bin/lib/ }
print "@libdirs\n";

__OUTPUT__
/usr/lib /lib /usr/local/lib
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The parentheses are required when combining an assignment if you wish to change the result in the leftmost variable. Normally, the result of a substitution is its success: either "" for failure, or the number of times the substitution was done. Contrast this with the preceding examples where the parentheses surround the assignment itself. For example:

($a =  $b) =~ s/x/y/g;      # copy $b and then change $a
 $a = ($b  =~ s/x/y/g);     # change $b, count goes in $a
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Update: made a Q&A node of it: How do I assign & substitute in one statement?