I think mirod has the most efficient algorithm, trying successively longer substrings. Another implementation: 
sub find_result {
   my $number = shift;
   my $prefix_length = length($number);
   my $result;

   1 until defined($result = $hash{substr($number, 0, $prefix_length--
+)})
      or $prefix_length <= 0;

   return $result;
}

[download]
This has the added benefit of allowing you to specify more specific mappings that take precedence over more generic. Another possibility might be constructing a tree of sorts, and walking the tree for each digit in your number: 
%hash = (
  1 => {
     2 => {
        3 => 1,
        4 => 2,
     },
     3 => {
        2 => 3,
     }
   }
);

# Equivalent to ( 123 => 1, 124 => 2, 132 => 3 )

sub find_result {
   my $number = shift;
   my $position = 0;
   my $pointer = \%hash;

   while (defined(my $current = substr($number, $position++, 1))) {
      return $current unless ref($pointer->{$current});
      $pointer = $pointer->{$current};
   }

   return;
}

[download]
Creating this new tree-shaped %hash from a list of prefix => result pairs is an exercise for the reader.

In reply to Re: Fast lookup for prefixes in hash by Fastolfe
in thread Fast lookup for prefixes in hash by Marcello

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.