$y could be passed as a reference to avoid the string copy further reduce the disadvantage, particularly for large strings.

my %dispatch=(
   A=>sub { split(/!/,$$_[0]); },
   ... 
); 

sub dispatch {
   my $x=$letters[random($nLetters)];
   my $y='xyzzy!' x random(1000);
   if (exists $dispatch{$x}) {
      $dispatch{$x}->(\$y);
   } else {
      warn "Huh?";
   } 
}
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On the other hand, maybe the parameter string isn't copied when the function references $_[0] directly?