There is common pattern to cache result of heavy calculations.

use strict;
use warnings;
use v5.10;

our $_cached;
sub somefunc
{
   $_cached //= do {
     say "Heavy calculations";
     42+($ARGV[0]||0);
   };
}

say somefunc();
say somefunc();
__END__
Heavy calculations
42
42
[download]

since 5.10 there is "state" statement, making this pattern even more easy.

use strict;
use warnings;
use v5.10;

sub somefunc
{
   state $var = do {
     say "Heavy calculations";
     42+($ARGV[0]||0);
   };
}

say somefunc();
say somefunc();
__END__
Heavy calculations
42
42
[download]

However I see one problem here: unit testing.

With first example it's easy to test somefunc with several possible inputs.

{
  local $_cached = undef;
  local @ARGV = (3);
  say somefunc();
}
{
  local $_cached = undef;
  local @ARGV = (5);
  say somefunc();
}
[download]

and with "state" it looks impossible? Once you call somefunc() from your test, it's result cached forever.

Looks like a missing important feature, which can encourage people not using unit testing? What do you think?

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