Is it possible to redefine subroutines to be localized for a part of the code?

Question

Is it possible to redefine the _function_used_by_exported_function only for the exported_function call in the second_routine?

#!/usr/bin/env perl
use warnings; 
use strict;
use Needed::Module qw(exported_function);


sub first_routine {
    return exported_function( 2 );
}

no warnings 'redefine';

sub Needed::Module::_function_used_by_exported_function {
    return 'B';
}

sub second_routine {
    return exported_function( 5 );
}

say first_routine();
say second_routine();

Answer 1

You can locally redefine the sub _function_used_by_exported_function inside your sub second_routine.

package Foo;
use warnings; 
use strict;
use base qw(Exporter);
our @EXPORT = qw(exported_function);

sub exported_function {
  print 10 ** $_[0] + _function_used_by_exported_function();
}

sub _function_used_by_exported_function {
  return 5;
}

package main;
use warnings; 
use strict;

Foo->import; # "use"

sub first_routine {
    return exported_function( 2 );
}

sub second_routine {
    no warnings 'redefine';
    local *Foo::_function_used_by_exported_function = sub { return 2 };
    return exported_function( 5 );
}

say first_routine();
say second_routine();
say first_routine();

I lifted the typeglob assignment inside sub second_routine from brian d foy's Mastering Perl, Chapter 10 on page 161. The sub is redefined by assigning to the typeglob, which only replaces the coderef part of it. I use local to only do that inside the current block. That way, the outside world is not affected by the change, as you can see in the output.

1051
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1051