So something like:

  my $cases = get_file('cases');
  eval qq@

     given ('somevar')
          {
           $cases
          }
    @;
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Ewww ugly!! Given how dog-awful slow eval is, I've done my utmost to completely remove it from my lexicon! I think there is only one place in my new system where it appears and then its only invoked if for some reason I have to embed raw Perl into my documents. (I believe that that can be completely avoided tbh)

Not sure I follow you altogether but it seems what you want to do is something like:

while(my $thing = input()){
  my $consequence = given_thing($thing,\&get_file,qw(foo bar baz));
## accept the consequences of your actions ...
}

sub given_thing {
my ($thing,$action,@keys) = @_;

    do { $thing ~~ $_ and return $action->($thing)} for @keys;

}
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I wouldn't be too quick to throw eval away. Sure it is slow and a security risk but the ability to dynamically create executable perl can significantly shorten the road from here to there.

I concur that eval has it's place, it's just that in most cases there exists a neat solution that can and really ought to be used instead.