I didn't know [3..5] construct

kcott's (localtime)[3..5] is a range operator used in a list slice.

if there is something yet simpler, what will replace all occurences (theoretically there can be more of them, although currently are not) of backticks pairs with standard output of their contents

I'm still unclear on what syntax you want to use here - the shell's, Perl's, or something custom? In bash's syntax, "X`date`Y" will interpolate, while in Perl's syntax it won't. And the shell quoting rules can get pretty tricky, for example "X`echo \\\``Y""1"'2' will evaluate to X`Y12.

Perhaps you could show several representative samples of the kinds of strings you want to interpolate, with some complex examples mixed in?