Don't do that. You're assigning a sub in Foo to return the results of a call made to ForeignModule. Then you're setting main::do_something to be the same as that subroutine. What you actually want is not to return the value but to make main::do_something the very same sub as ForeignModule::NEXT.

package ForeignModule;
sub NEXT { print scalar caller }
1;
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package Foo;
# there is no do_something
sub import {
    my $target = caller;
    no strict 'refs';
    *{"$target:\:do_something"} = \&ForeignModule::NEXT;
}
1;
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package main;
use strict;
use warnings;

use Foo;
do_something();
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