How does one substitution with capture compare to doing 2 substitutions without capture?

In your example:

        $hr_returnvalue->{$CurrentColumnName} =~ s/^\s*(.*?)\s*$/$1/; 
+   # Trim whitespace
[download]

The regular expression /^\s*(.*?)\s*$/ will always match, regardless if there is or is not whitespace present, so the substitution will always be done.

In my example:

        s/^\s+//, s/\s+$// for $hr_returnvalue->{$CurrentColumnName}; 
+   # Trim whitespace
[download]

The regular expressions /^\s+/ and /\s+$/ will only match if there is whitespace present and so the substitution will only be done on the occurrence of whitespace.

Running a benchmark would be good start, and you should try to use data similar to, or the actual data that your program uses.