As always, ikegami provides a clear (and clearly explained), effective solution.

Here's another approach that has the advantage of a bit more concision (I doubt it will actually run any faster), but the drawback that it uses some Perl idioms that may be unfamiliar to the programmer/maintainer. You can leave out the  defined test if you're absolutely sure a 'gene' will never be anything that looks false to Perl. (A similar problem, unaddressed, exists for the taxons.) 

>perl -wMstrict -le
"use List::Util qw(first);
 my %gg = (
   1567 => [ qw(NP_011  NP_012  NP_013  NP_025)         ],
   2000 => [ qw(NP_020  NP_021  NP_024  NP_025  NP_035) ],
   9999 => [ qw(NP_900  NP_910  NP_902  0)              ],
   9998 => [],
   );
 SEARCH:
 for my $gene_to_find (qw(NP_011  NP_025  NP_999  0)) {
   print qq{searching for gene '$gene_to_find':};
   my $matching_taxon =
     first { defined first { $_ eq $gene_to_find } @{ $gg{$_} } }
     keys %gg
     ;
   my @all_matching_taxons =
     grep { defined first { $_ eq $gene_to_find } @{ $gg{$_} } }
     keys %gg
     ;
   print qq{  NOT found} and next SEARCH if not $matching_taxon;
   print qq{  first found: '$matching_taxon'};
   my $all_found = join q{' '}, @all_matching_taxons;
   print qq{  all found: '$all_found'};
   }
"
searching for gene 'NP_011':
  first found: '1567'
  all found: '1567'
searching for gene 'NP_025':
  first found: '2000'
  all found: '2000' '1567'
searching for gene 'NP_999':
  NOT found
searching for gene '0':
  first found: '9999'
  all found: '9999'

[download]

See List::Util.

In reply to Re^2: how to access hash key from the hash value when hash key is pointing to an array of hash values? by AnomalousMonk
in thread how to access hash key from the hash value when hash key is pointing to an array of hash values? by BhariD

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