I'm returning it in a function call...

In case the string is huge and you want to avoid some temporary memory allocation (which could push up the peak memory usage of the program), you could return a reference to the scalar:

#!/usr/bin/perl

sub mem {
    print "$_[0]:\n";
    system "/bin/ps", "-osize,vsize", $$;
}

sub foo {
    mem(0);
    my $data;
    $data .= "xxxxxxxxxx" for 1..1_000_000;
    mem(1);
    return \$data;  # return reference
}

my $r = foo();  # then get at the data using $$r
mem(2);

__END__
$ ./880173.pl
0:
   SZ    VSZ
  612  19804
1:
   SZ    VSZ
10468  29660
2:
   SZ    VSZ
10468  29660
[download]

Using the same snippet, but with return $data, you'd get:

0:
   SZ    VSZ
  612  19804
1:
   SZ    VSZ
10468  29660
2:
   SZ    VSZ
20236  39428
[download]

As you can see, the process size has grown additionally by about the size of $data.

Whether this really saves anything overall depends on what else the program is doing to actually print the string, etc.