That depends on what you mean by 'compile once'. If you mean, once per invocation of the program, it's not too terribly difficult to do. Simply build a text string that can be read as valid Perl code containing an anonymous subroutine:

my $sub <<SUB;
sub {
    print "I've been compiled once!  I'm anonymous!\n";
}
SUB

my $sub_ref = eval $sub;
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This is off the top of my head, so you may have to play around with it slightly.

If you're looking for a way to compile a string once and have that persist across invocations of the program, you'll have to look into something like B::Bytecode, which is experimental and may not gain you very much.

Other options include normal anonymous subroutines and closures. There's probably a way to get around most specific problems there, but knowing how to eval an anonymous subroutine into existence can occasionally come in handy.