This avoids loops (external to the regex at least). It might compare favorably if you remove the balance check code.

sub peeln {
    local $_ = shift;
    my $n = shift;
    my($o, $c) = (tr[(][], tr[)][]);
    warn 'Unbalanced parens' unless $o == $c;
    $n = $c if $n == -1 or $n > $c;
    m[^ (?: .*? \( .*?){$n} (.+) (?: .*? \) .*?){$n} $]x;
    $1;
}

my @tests = (
    'hello(what(is(this(all(about)))))',
    'hello(what(is(this(all(about))))))',
    'hello((what(is(this(all(about)))))',
    'A(B(c) D(e) f(g(h(i))))',
);

for my $test ( @tests ) {
    print "$test : $_ : ", peeln( $test, $_ ) for -1 .. 5;
}
__END__
P:\>270595
hello(what(is(this(all(about))))) : -1 : about
hello(what(is(this(all(about))))) : 0 : hello(what(is(this(all(about))
+)))
hello(what(is(this(all(about))))) : 1 : what(is(this(all(about))))
hello(what(is(this(all(about))))) : 2 : is(this(all(about)))
hello(what(is(this(all(about))))) : 3 : this(all(about))
hello(what(is(this(all(about))))) : 4 : all(about)
hello(what(is(this(all(about))))) : 5 : about

Unbalanced parens at P:\270595.pl8 line 28.
Use of uninitialized value in print at P:\270595.pl8 line 42.
hello(what(is(this(all(about)))))) : -1 :
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 0 : hello(what(is(this(all(about)
+)))))
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 1 : what(is(this(all(about)))))
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 2 : is(this(all(about))))
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 3 : this(all(about)))
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 4 : all(about))
Unbalanced parens at P:\270595.pl8 line 28.
hello(what(is(this(all(about)))))) : 5 : about)
Unbalanced parens at P:\270595.pl8 line 28.

hello((what(is(this(all(about))))) : -1 : all(about
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 0 : hello((what(is(this(all(about
+)))))
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 1 : (what(is(this(all(about))))
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 2 : what(is(this(all(about)))
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 3 : is(this(all(about))
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 4 : this(all(about)
Unbalanced parens at P:\270595.pl8 line 28.
hello((what(is(this(all(about))))) : 5 : all(about
Use of uninitialized value in print at P:\270595.pl8 line 42.

A(B(c) D(e) f(g(h(i)))) : -1 :
A(B(c) D(e) f(g(h(i)))) : 0 : A(B(c) D(e) f(g(h(i))))
A(B(c) D(e) f(g(h(i)))) : 1 : B(c) D(e) f(g(h(i)))
A(B(c) D(e) f(g(h(i)))) : 2 : c) D(e) f(g(h(i))
A(B(c) D(e) f(g(h(i)))) : 3 : e) f(g(h(i)
A(B(c) D(e) f(g(h(i)))) : 4 : g(h(i
Use of uninitialized value in print at P:\270595.pl8 line 42.
A(B(c) D(e) f(g(h(i)))) : 5 :
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It doesn't handle (?term?) nesting like this a(b(c) d(e)), but it was unclear to me what the return would be in these circumstances?