use if $ENV{SLICE}, 'Slicer';
use if !$ENV{SLICE}, 'constant', slice => 0;

##</code><code>##

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;

##</code><code>##

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;

##</code><code>##

>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 :!