You shouldn't ever be using lexicals with (?{...}) or (??{...}) per ysth's note. That is almost always wrong. Also, your expression (??{ $skip }) is needlessly verbose and wasteful of runtime. Just interpolate $skip like qr/ ... $skip ... /x.

Your code can be implemented with proper recursion in 5.10:

{
    my $skip = qr/\s*/;
    my $expr = qr/ (?<EXPR>

                     # term
                     $skip
                     (?<TERM> [a-zA-Z]+
                       | [1-9]\d*
                       | \( (?&EXPR) $skip \) )
                     # term*
                     (?: $skip [-+*/] (?&TERM) )* )
    /x;
    my $re = qr/^ $expr $skip \z/x;
    sub is_valid { shift =~ /$re/o }
}
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