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

##</code><code>##

@a = grep { /...$xxx.../ } @b

##</code><code>##

my $re;
...
{
   $re ||= qr/.../;
   ...
}

##</code><code>##

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