If you're doing this in ways where a given input token needs to broken up into one (or more) output tokens, why not do simple matching or splitting? i.e.

my @captures = ($input_token =~ /$regex/);
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Since you know the bits you want to replace already, it'd become something akin to:

my $repl_index = 1;
{ 
    print "attempt 1 ";
    my $str = "http://www.domain.com/";
    my $regex = qr|http://(.*)\.(.*)\.(.*)|;
    my @captures = ($str =~ /$regex/i);
    print "yields: ", $captures[$repl_index], "\n";
}
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