The purpose of the o modifier was to speed up dynamically built regexps. The downside is that it creates hard to find bugs since any change to the regexp gets ignored. qr// was introduced as an alternative to the o modifier. There isn't any reason to use the o modifier any more.

If all you want to do is avoid having $re compiled for every @b element, you don't need the o modifier:

my $re=qr/...$xxx..../;
@a = grep { /$re/ } @b
[download]

There are checks in place to avoid recompiling a regexp is the interpolated variables haven't changed, so the following also does what you want:

@a = grep { /...$xxx.../ } @b
[download]

If you want the o behaviour:

my $re;
...
{
   $re ||= qr/.../;
   ...
}
[download]

If you want a cache:

my %re_cache;
...
{
   my $re = $re_cache{$key} ||= qr/.../;
   ...
}
[download]

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