sub x {
   my $func = do { no strict 'refs'; *{caller() . '::_x'}{CODE} };
   die() unless defined $func;
   &$func;
   # or &$func(...);
}

##</code><code>##

package M;

use strict;
use warnings;

BEGIN {
   our ISA 'Exporter';
   our EXPORT_OK qw ( x );
   require Exporter;
   *import = \&Exporter::import;
}

sub x {
   my $func = do { no strict 'refs'; *{caller() . '::_x'}{CODE} };
   die() unless defined $func;
   &$func;
   # or &$func(...);
}

1;

^^^   M.pm    ^^^
vvv script.pl vvv

use strict;
use warnings;

use M qw( x );

sub _x {
   print("main::_x(" . join(', ', @_) . ")\n");
}

x('a', 'b');

__END__
outputs
=======
main::_x(a, b)

##</code><code>##

package M;

use strict;
use warnings;

BEGIN {
   our ISA 'Exporter';
   our EXPORT_OK qw ( x );
   require Exporter;
   *import = \&Exporter::import;
}

sub x {
   my $callback = shift(@_);
   &$callback;
   # or &$callback(...);
}

1;

^^^   M.pm    ^^^
vvv script.pl vvv

use strict;
use warnings;

use M qw( x );

sub _x {
   print("main::_x(" . join(', ', @_) . ")\n");
}

x(\&_x, 'a', 'b');

__END__
outputs
=======
main::_x(a, b)