You could write a sub that would create the temp variable for you and return it:

#!/usr/bin/perl -w

use strict;

foreach my $str (qw(fish apple license gradual canaan))
{
  print sr($str,'s/([aeiou])/\u$1/g'),"\n";
}

sub sr
{
  my($in,$re)=@_;
  eval "\$in =~ $re;";
  $@ and die "Bad regex '$re'\n";
  $in;
}
[download]

although a temp variable is still being created, just not explicitly.

This is probably the best you're going to be able to do (although there's probably a way to do it without eval), since doing something nondestructively means that the original must be kept intact, so there have to be two copies of the variable.