#! perl -w
package mylib;
my ($MODULE_PATH, $CONF_PATH);
($CONF_PATH = __FILE__) =~ s/pm$/conf/i;
use constant {
    MODULE_PATH => __FILE__,
    CONF_PATH => $CONF_PATH
};
print CONF_PATH;

##</code><code>##

#! perl -w
package mylib;
my ($MODULE_PATH, $CONF_PATH);
# assign value at compile-time before constants
BEGIN {
    ($CONF_PATH = __FILE__) =~ s/pm$/conf/i;
}
use constant {
    MODULE_PATH => __FILE__,
    CONF_PATH => $CONF_PATH,
};
print CONF_PATH;