It is arguably more readable to save off the old value, and re-instate it if func returns undef/zero:

my $oldval;

$val = func( $oldval=$val) || $oldval;
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Newer perl's allow the alternative of using the // (defined or) operator.

To maintain the challange of "not creating a new variable", one could try:

  $val = func( $_=$val) || $_;
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Add a BLOCK, if $_ is being abused:

    $val = do{ func( local $_=$val) || $_ };
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