Try this with both versions of make_grep:

$a = make_grep("a");
$b = make_grep("b");
print &$a(qw(ack thpt barf)), "\n";
print &$b(qw(ack thpt barf)), "\n";
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With the eval, you get "ackbarf" and "barf", which is correct. Without it, you get "ackbarf" twice. String eval compiles a new sub. Without the eval, the two subs share the same compiled regex. Once the /o causes it to lock down, it is locked for both subs -- even though they see different values of $pat.

This technique is kind of nasty. Use qr//, like suaveant says. Be sure to compile the regex in the right place.

sub make_grep {
  my $pat = shift;
  my $re = qr/$pat/;
  return sub { grep /$re/, @_ }
}
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