This is answered in Optimizing pattern evaluation in perlretut:

If $pattern won't be changing over the lifetime of the script, we can add the //o modifier, which directs Perl to only perform variable substitutions once

It wouldn't make sense to use this optimization in the context you've shown - even if it were embedded in a loop, there is no variable being interpolated.

A point on formatting posts - please do not use <pre> tags. Rather, write normal text, separate paragraphs with <p> tags and wrap your code in <code> tags. See How do I post a question effectively?.