use 5.010;

my $standard_say = sub {
  say 0 if $x ~~ [0..9,   50..59];
  say 1 if $x ~~ [10..19, 60..69];
  say 2 if $x ~~ [20..29, 70..79];
  say 3 if $x ~~ [30..39, 80..89];
  say 4 if $x ~~ [40..49, 90..99];
};

$standard_say->(40);

# Now imagine we decide to special-case 39...
my $modified_say = sub {
  if ($_[0] == 39) {
    say "yay!";
    return;
  }
  $standard_say->(@_);
};

$modified_say->(40);
$modified_say->(39);