# Save a reference to the original variable:
*nothingToSeeHere::var= \$var;
# Replace the variable with a new variable instance and assign
# that new instance the given value (or undef if none mentioned):
*var= \undef; # (but see footnote)
# Restore the previous instance (and thus the previous value):
ON_SCOPE_EXIT { *var= \$nothingToSeeHere::var; }