my $target = 'this is a test of the servise'; 
 
if ($target =~ /test/) { 
    $target =~ s//test2/; 
} 
 
if ($target =~ /nothing/) { 
    $target = s//null/; 
} else { 
    $target =~ s//foo/; 
}

##</code><code>##

               [....] If the pattern evaluates to the empty
               string, the last successfully executed regular
               expression is used instead. [....]