Based on the little you've told us, you could do:

...
   my @selectedKeys = grep{ $hash{ $_ } eq $scalar } keys %hash;
...
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But doing that, ie. looking up keys in a hash from the values, defeats the purpose of using a hash.

The chances are that you should really be building the hash the other way around.

Instead of

$hash{ $key } = $value;
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push @{ $hash{ $value } }, $key;
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Then, when you need the 'keys' associated with a particular 'value' you find them directly:

...
   my @selectKeys = @{ $hash{ $scalar } };
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thus avoiding a potentially costly linear search.

If you were to describe the bigger picture of your code, and post the code you have so far, rather than just this tiny fragment of requirement, then you'd probably get not just a solution to the small problem, but good advice on solving the bigger problem more effectively.

So my general problem is one in bioinformatics: given the entire proteome of a species, where each gene can be spliced into multiple protein isoforms of different lengths, pick only the longest protein isoform for each gene

In other words, if there are genes A, B, C etc.

with respectively 3, 1 and 2 protein isoform(s)

a.1, a.2, a.3, b.1, c.2, c.5, c.7, and

of respective lengths 12, 11, 12, 15, 34, 12, 45, and

and with their corresponding peptide sequences,

then, I want the PERL script to return ONLY a.1 or a.3 for gene A and its corresponding sequence, b.1 and its sequence for gene B, c.7 and its sequence for gene C... you get the idea?

Your suggested syntax using grep worked, except that it returned all the matches into an array, so I used shift to gather the first match (which for my purposes is the same as any other matches, if multiple keys are present as matches to my values in the array)

Thanks for the useful syntax, I had not come across it yet in the Beginning Perl 3rd edition book, and its only been 10 days since I started teaching myself PERL! So your help is much appreciated...

I can this my "non-redundification" PRL script that removes protein isoform redundancy by selecting ONLY the longest isoform for each gene in a proteome. My script is ~100 lines long, which I think would be a joke for you Monks! But hey, I am just a Padawan learner as of now! :)

Thanks again to both of you!

If all you want is one, then I'd suggest List::Util::first().

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

I agree with BrowserUk that we may be able to provide better answers given a better problem definition. This sounds a bit like an XY Problem.

However, an alternative approach is to 'invert' the hash. This will give very fast lookup of a key for a value in the original hash, but will only ever return one single key, the same one every time, chosen at random, from the set of all keys in the original hash having a particular value. (Note that the 'random choice' referred to here occurs when the original hash is inverted, and is fixed thereafter.)

>perl -wMstrict -le
"my %hash = qw(one 1  uno 1  un 1  two 2  tres 3);
 my %hsah = reverse %hash;
 ;;
 for my $v (1, 2, 4) {
   print qq{value '$v' },
         exists $hsah{$v} ? qq{is $hsah{$v} } : q{does not exist },
         q{in original hash};
   }
"
value '1' is uno in original hash
value '2' is two in original hash
value '4' does not exist in original hash
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