The underlying structure doesn't permit this to be done without iterating over every key until the condition is matched.

If there's a possibility of multiple matches, 

if ( $incomplete_isbn =~ m/^\d{5}$/ ) {
    my $re_key = qr/.+$incomplete_isbn.+/;

    my @matching_keys = grep /$re_key/,
                        keys %$hashref_isbn;

    ...
}

[download]

But it sounds like you want to check the same digits of the ISBN every time, and that those digits are unique. If so, then your hash is incorrectly keyed. It should be keyed by the partial ISBN. The full ISBN can be included as part of the value. 

my %titles = (
   partial_isbn => [ full_isbn, title ],
   partial_isbn => [ full_isbn, title ],
   partial_isbn => [ full_isbn, title ],
   partial_isbn => [ full_isbn, title ],
);

[download]

By the way, qr/.+$incomplete_isbn.+/ doesn't seem to match what you specified, and can be simplified to qr/.$incomplete_isbn./.

By the way, $hashref_isbn is an aweful name. What does the hash contain, a list of titles? Then it should be called "titles" or "titles by isbn", not "isbn".

By the way, Tie::Hash::Regex does what you say you want, but I don't think it's a good solution to your problem.

Updated due to accidental post.

In reply to Re: Possible to use an expression as the hash key with exists? by ikegami
in thread Possible to use an expression as the hash key with exists? by wenD

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.