Does this mean I have to implement module called 'slice' and then implement the unimport method. Is that what you meant ? I haven't done 'no'-module !

I meant no Slicer 'SomeModule';, so you would create an unimport function instead of of unslice.

The constant will override the Slicer.pm definition

It will do that even if slice has no prototype. I notice you get a warning when you override, and two if there's a prototype mismatch, so may I suggest

use if $ENV{SLICE}, 'Slicer';
use if !$ENV{SLICE}, 'constant', slice => 0;
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or better yet, move that logic into Slicer:

package Slicer;

use base Exporter;
our @EXPORT = qw( slice );

sub _slice {
    my @caller = caller;
    print $caller[0];
    print ":: sliced :" , @_;
    return 1;
}

if ($ENV{SLICE}) {
   *slice = \&_slice;
} else {
   require constant;
   import constant slice => 0;
}

1;
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Update: Nevermind, still doesn't work for slice(...). You really should use two functions instead of requiring people to use &.

package Slicer;

use base Exporter;
our @EXPORT = qw( slice sliced );

use constant slice => $ENV{SLICE};

sub sliced {
    return unless slice;

    my @caller = caller;
    print $caller[0];
    print ":: sliced :" , @_;
}

1;
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>set SLICE=

>perl -wle"use Slicer; print('!') if slice;

>perl -wle"use Slicer; sliced('!');

>set SLICE=1

>perl -wle"use Slicer; print('!') if slice;
!

>perl -wle"use Slicer; sliced('!');
main:: sliced :!
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