"Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems."

Perl’s documentation is far more clear about the consequences:

(https://perldoc.perl.org/perlop#Regexp-Quote-Like-Operators)

   o   Compile pattern only once.

  […]

  PATTERN may contain variables, which will be
  interpolated every time the pattern search is
  evaluated, except for when the delimiter is a
  single quote. […] Perl will not recompile the
  pattern unless an interpolated variable that
  it contains changes. You can force Perl to skip
  the test and never recompile by adding a /o
  (which stands for "once") after the trailing
  delimiter. Once upon a time, Perl would recompile
  regular expressions unnecessarily, and this
  modifier was useful to tell it not to do so,
  in the interests of speed. But now, the only
  reasons to use /o are one of:

  [reasons]

  The bottom line is that using /o is almost
  never a good idea.

  However, mentioning /o constitutes a promise
  that you won't change the variables in the
  pattern. If you change them, Perl won't even
  notice.

(https://github.com/Perl/perl5/blob/perl-4.0.00/perl.man#L272...)

  PATTERN may contain references to scalar
  variables, which will be interpolated
  (and the pattern recompiled) every time the
  pattern search is evaluated. […] If you want
  such a pattern to be compiled only once, add
  an “o” after the trailing delimiter. This
  avoids expensive run-time recompilations, and
  is useful when the value you are interpolating
  won't change over the life of the script.