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

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

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

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