But that produces the error "Use of uninitialized value $_ in exponentiation (**) at coderef.pl line 19."

The problem is your use of $_.

You pass it to the coderef as an argument, but you never set it!

And within the coderef -- ie. subroutine -- the first argument passed is aliased to $_[0] as with all subroutines.

To be able to use $_ within the coderef, you need to set it before calling the block. Try:

sub foo(&@) {
    my $coderef = shift;
    my @ret;

    for( @_ ) { ## let the for loop do the localisation and aliasing
        push @ret, $coderef->(); ## No arg
    }
    return @ret;
}
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