It seems to me that the first part, using with to topicalize is exactly what for does:

#!/usr/bin/perl -w
use strict;

my $foo = "a string";

print "before with: $foo\n";
for($foo) {
  s/a /not a /;
}
print "after with:  $foo\n";
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I believe that's indeed one of the reasons for the use of for as a keyword, instead or just foreach.