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@bindirs = qw( /usr/bin /bin /usr/local/bin );
for (@libdirs = @bindirs) { s/bin/lib/ }
...

__OUTPUT__
/usr/lib /lib /usr/local/lib

($a =  $b) =~ s/x/y/g;      # copy $b and then change $a
 $a = ($b  =~ s/x/y/g);     # change $b, count goes in $a