If I understand you correctly, you want to say

$str = "the boy walked the dog";
$str =~ s/walked/fed/ =~ s/boy/girl/ =~ s/dog/Audrey II/; # error
[download]

Here's why it shouldn't work. Since s/// returns a number of times a match is found, anyway you associate =~, you end up with something like

$str =~ 1
[download]

at some point. This is not legal. If you want to chain =~, then we must expect s/// to return a string.

Specifically, we must get one that does regular expressions such as s/this/that/ or m[^/usr/local/bin], but then this prohibits us from saying

if ($str =~ s/boy/girl/) {
    # do this
}
[download]

because the condition will evaluate to false. I'd rather have if ( $s =~ /match/ ) { .. } rather than the chain-ability of s///.

Update: Above discussion assumes that =~ is right associative, but in fact it is left associative. This means that as long as =~ returns an integer, we have a trouble (say 1=~ s/this/that/). I like roju's idea of returning a string, but I don't know how easy/hard it is to implement it.