$string = "firstword secondword";
print "original: $string\n";
# doesn't work
$search = '(second.+)';
$replace = '$1 thirdword';
($varsub = $string) =~ s/$search/$replace/;
print "variable substitution: $varsub\n";
# does work
($direct = $string) =~ s/(second.+)/$1 thirdword/;
print "direct: $direct\n";