constant.pm is pretty crufty by now, but here's a "clean-room implementation" of the same sort of idea...

use v5.12; use warnings;

BEGIN {
    package MyConstant;
    $INC{'MyConstant.pm'} = __FILE__;
    sub import {
        my $caller = caller;
        my $class  = shift;
        
        if (@_ == 1 and ref $_[0] eq 'HASH') {
            while (my ($key, $value) = each %{$_[0]}) {
                $class->_define($caller, $key, $value);
            }
        }
        else {
            $class->_define($caller, @_);
        }
    }
    sub _define {
        my $class = shift;
        my ($caller, $name, @values) = @_;
        my $code;
        
        if (@values == 1) {
            my $v = $values[0];
            $code = sub () { $v };
        }
        else {
            $code = sub () { @values };
        }
        
        no strict 'refs';
        *{"$caller\::$name"} = $code;
    }
}

use MyConstant {
    FOO => 4,
    BAR => 2,
};

say FOO . BAR;
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