If you are going to be making this query frequently, you may want to pre-build a "reverse lookup" hash:

 my %h1 = qw (1  a 2 b 3 a 4 c); 
 my %h2; #Used for Value-to-key  lookup

 push @{$h2{$h1{$_}}},$_ for keys %h1; # Builds the reverse hash

 print join( ",",@{$h2{a}});  #  Lookup and print all keys in h1 that 
+contain the value 'a'
# Prints: 1,3
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