BTW:URL encoding is easily applied after the interpolation.

Not without possible data loss.

my $value = 'foo&bar=baz';
my $key   = 'something'

my $url1 = encode_url("http://example.com/script?$value=$key");
my $url2 = 'http://example.com/script?%s=%s', map encode_url_chunk($_)
+, $key, $value;
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For proper working functions encode_url and encode_url_chunk (which have to encode different things, of course) the outcome will be different.