Why not just pass a reference to the callback sub?

package M;
use strict;
require Exporter;
our @ISA = qw[ Exporter ];
our @EXPORT = qw[ x ];

sub x{
    my( $sub ) = @_;
    &$sub;
}
1;
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#! perl -slw
use strict;
use M;

sub _x{ print __PACKAGE__ . "_x() called" }

x( \&_x );

__END__
P:\test>393046
main_x() called
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