@a_keys = grep { $myhash{$_} eq 'a' } keys %myhash;
[download]

%myhash = { 1 => 'a', 2=> 'b', 3=> 'a', 4=>'c'}
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for (keys %myhash) {
    print "$_ eq a\n" if $myhash{$_} eq 'a';
}
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Or, to avoid the extra hash lookups, iterate over key-value pairs using each.

my @as = ();
while (my ($key, $value) = each %myhash){
   push @as, $key if $value eq 'a';
}
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Update:

Is there a monk among us who can shorten this into a one-liner, similar to the grep keys examples we have seen?

It seems that each does not combine nicely with grep, map and their ilk.

Voilà:

%newhash = reverse do { my $hash = reverse %myhash; %hash };
[download]

To get the keys only (which are the values in the reversed hash:

@unique_keys = do { my $hash = reverse %myhash; values %hash };
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update: oops, got it all wrong. It' about getting all keys where duplicate values are found. As a single statement, formatted for readability

%myhash = (1 => 'a', 2=> 'b', 3=> 'a', 4=>'c',5 => 'b', 6 => 'a');
@k = do {
        my %s;
        map {
            my $v=$_;
            grep { $myhash{$_} eq $v} keys %myhash
        } grep {$s{$_}++==1} values %myhash
};
print "$_ => $myhash{$_}\n" for @k;
__END__
6 => a
1 => a
3 => a
2 => b
5 => b
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Ah well. Readability... ;-)

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

I tried doing that, It is working.

use Data::Dumper;
%myhash = ( 1 => 'a', 2=> 'b', 3=> 'a', 4=>'c', 5 => 'a', 6 => 'a', 7 
+=>'b');
%newhash = map { ($_,$myhash{$_}) }  grep { $myhash{$_} eq 'a'; } keys
+ %myhash ;
print Dumper \%newhash;
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 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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Hi isha,

If you want to manipulate the simple hash, then your hash will be like this,

To manipulate this simple hash, you can do like

use strict;

my %myhash = ( 1 => 'a', 2=> 'b', 3=> 'a', 4=>'c');
for my $key (keys %myhash){
    if($myhash{$key} eq 'a'){
        print "KEY:$key\n";
    }
}
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But you created the hash as

my %myhash = { 1 => 'a', 2=> 'b', 3=> 'a', 4=>'c'};
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which is anonymous hash reference. To manipulate this hash reference, then you can do like

use strict;
my $myhash = { 1 => 'a', 2=> 'b', 3=> 'a', 4=>'c'};
for my $key (keys %{$myhash}){
    if(${$myhash}{$key} eq 'a'){
        print "KEY:$key\n";
    }
}
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Punitha

my %val_by_key = (
   1 => 'a',
   2 => 'b',
   3 => 'a',
   4 => 'c',
);

my %keys_by_val
foreach my $k (keys %vals_by_key) {
   my $v = $vals_by_key{$k};
   push @{ $keys_by_val{$v} }, $k;
}

# a: 1, 3
print('a: ', join(', ', @{$keys_by_val{'a'}}), "\n");

# a: 1, 3
# b: 2
# c: 4
foreach my $v (keys %keys_by_val) {
   print("$v: ", join(', ', @{$keys_by_val{$v}}), "\n");
}
[download]

See question "How do I look up a hash element by value?" in perlfaq4.