sub foo { print($__KLUDGE::phase eq 'END' ? 'END' : 'not END', $/); }

# Needs to be at the beginning of the program to be reliable.
BEGIN { $__KLUDGE::phase = 'BEGIN'; }

# Needs to be at the end of the program to be reliable.
END { $__KLUDGE::phase = 'END'; }

# ...

END { foo(); }

# ...